The value of startup equity can be estimated but takes a bit of work. You basically have to run a bunch of scenarios and estimate the likelihood of that scenario and your gain from the scenario. A 10% chance of a $1000 win is pretty dull but 30% chance at a $1,000,000 win sounds much better.
A good startup will have a business plan, a valuation strategy, some key milestones and some credible plan for equity. Research those carefully. You can also look at comparable companies and see how they are doing and what's their valuation is. Take a look at the investors: A highly reputable venture capital firm would be a good sign, since they do a pretty careful assessment before they open up the wallet.
This way you can build up a picture of the potential upside and the associated risk. The risk will always be there and one of the scenarios always will be "you get diddly squat". That is something you would need to accept.
Valuing at zero without doing the homework is just plain stupid. While in many cases this may be the outcome , there are also quite a few cases out there where equity is highly valuable and it would be silly to not at least give it a thorough evaluation.