Here are some questions you can ask yourself when trying to decide if you should get an optional certificate or not:
Things to consider across your industry - these answers should be true across a discipline or type of business. They typically evolve over time, but slowly.
How often is the certificate requested in job descriptions?
If a certificate is mentioned in a large percentage of job descriptions, then naturally getting the certificate will increase the number of jobs you can apply for, therefore increasing your changes at getting a job in the industry you've chosen.
How often is the certificate requested in customer requests/contracts?
In many cases, a job description including a request for a certificate represents an ongoing demand on the part of customers that employees have this certificate. Perhaps the goes beyond "optional", although in industries where certification practices are evolving, this is a likely case, and if it continues as an increasing trend over time, it just may become a required certificate.
Is the certificate widely recognized within your industry?
It's very unlikely a certificate is worth getting if nobody in the industry has ever heard of the certificate before. Be sure you check that the certificate is one that is recognized within the industry.
Is the certificate viewed negatively by the industry in your area?
Some certificates have a negative reputation, and can actually be harmful to your job prospects. This is usually a problem localized to a small geographic area or within an agency, however it can happen if the accrediting institution has a poor reputation, teaches substandard information, or is known to certify unqualified people.
Is there a pay differential associated with this certificate?
Note, this is both industry related and personally focused. Most of the time, an optional certificate won't have an industry-wide standard of higher pay, but there may be trends. Sometimes even a salary search site can tell you whether there's any large scale data here.
Are there ongoing benefits?
Networking opportunities, certificate-holders-only materials and information, special deals - there can be ways to improve yourself post-certificate that are less obvious than direct salary increases.
Things that are unique to each person.
How much relevant work experience do you have?
Certificates are often used to make up for a lack of work experience. If you have very little relevant work experience, a certificate is a great way to show you have some knowledge and/or interested related to the job.
However if you have plenty of related work experience to show off during an interview, than a certificate is less likely to be the deciding factor between if you get a job or not.
Are you actually looking for a job?
The main point of most optional certificates is to increase your marketability when searching for a job. If you already have a job and are not seeking another job, an optional certificate is unlikely to be as much of value to you.
How difficult is the accreditation to obtain?
The hardest won skills usually have the highest overall value. Similarly, some certifications have a reputation for being challenging, with high standards, a high "fail rate" and the need for retakes. Others serve more to demonstrate that you attended a course. One benchmark for this is to find out if significant "home study" outside of the course is usually required to pass and gain accreditation.
Can you ask for a higher salary just because you have this certificate?
In most cases, probably not, but sometimes you can request a higher pay rate if you have more education, so don't forget to look into this if it applies. Also, keep in mind that the trends on a salary site and your personal experience are slightly different. Just getting the certificate may not be the reason that anyone gets a pay increase - but many people who get a valuable certificate become more highly paid because they use that valuable knowledge to improve their work.
Do you have an educational gap?
Sometimes an extra certificate can bridge an educational gap. There can be cases where there's a traditional path through a career, but with enough hands on experience, the educational side doesn't matter. That said, some certificate programs can help fill the gap of knowledge areas that aren't really taught on the job, but are assumed to be understood. I see this particularly in technical areas that change over time, or fields where there is no one true way.
Does the subject matter of the certificate actually interest you?
Since this question is specifically about optional certificates, I would not recommend wasting time/money on a piece of paper if you don't have any interest in the subject at all.
Barriers to Entry
Things to consider that make the certificate harder to get. Sometimes a relatively useless looking certificate may be worthwhile if it is cheap and easy to get and has good potential.
How expensive is the certificate?
Some certificates are simply not worth the price. Before spending a lot of money on a certificate, be sure the job that certificate will help you get is one that pays enough to justify the certification cost.
How much work will the certificate take?
Certificate programs can be as simple as taking a test (if you already have the knowledge) or as hard as a year or more of study with homework every week. It's a mix of technical material and the individual's existing experience to say for sure how many man-hours a certification process will take. But bear in mind that there are definitely differences between certificates that are a pass/fail test vs. certificates based on a body of study and coursework. Both what you get out of them, and what you put in can vary greatly.
How expensive is sustaining the certificate?
Some certificate programs can have a sustaining requirement. In technical areas, for example, many certifications can require proof of continuing education. That my improve the reputation of the certificate program, but it also leverages an ongoing requirement on the certificate holder. Not to mention the cost and frequency of renewel fees.
Have you checked with co-workers about this certificate?
In a team environment, it may be worth asking co-workers for their opinion of the certificate. If most have it then that could be a clue that it is common and worth having to better blend into the team. On the other hand, this could be a way to bring something new into an organization. This is similar to the recognized/viewed negatively questions but a bit more specific to within an organization.
I would also recommend reading through the answers to the question Are certifications worth it? on Programmers.SE. It provides some other insight on certificates, although it is speaking specifically about programming certificates, and not certificates in general.
But last of all, don't forget the deciding factor is You. If you really want a certificate, don't let any of these factors affect your decision. Go for it!
This is a community wiki answer because I would like the community to help keep this answer comprehensive, accurate, and up-to-date. If you feel you can add anything to it, please feel free to edit it and include additional information.