It depends on the industry you are going in. I don't think bad credit = bad workers. In all cases, it's a point of possible failure that can be avoided. Many industries are putting exceptions in given the recent recession. The government, for example, now considers some credit card debt as okay so long as you've been making payment. In the past, any credit card debt that wasn't paid back the same month, was a automatic red flag. The historical trend is that not every employee with bad credit is bad, but nearly all employees who went against the government or banks tend to have made poor financial decisions. For example,
- Government - Given historical information of past employees who went against the government, they typically have money problems or bad spending habits. By having poor credit, it's a indicator that you might sell off information in hopes of meeting those debts. It is also a good blackmail/bribery subject if you try to keep it hidden - from say your spouse because you spent the life savings - so you might try to "hush" people by stealing information from the government in exchange of them not disclosing your bad habits.
- Banking - By having bad credit or a history of not paying debt, it means you might be tempted to cheat or "shave" off the top of other people bank or payments. Again, banks are going off historical information on past employees who cheated them, and people who owe debt or want to pay off large debts tend to cheat the system in hopes to get money to pay the debt. For example, using a bank card and writing off a rent payment as some bogus company expense.
- Fire/EMT/Police - I wanted to volunteer for EMT a while back in college and during the first meeting, they told us they'd do a full background and credit check and at least one person from the class dropped out when they failed the credit check. I ultimately couldn't do the work because of the time constraint wasn't working with my schedule. The consideration here is that you'll be dealing with people in vulnerable positions and you may be tempted to "borrow" if you notice that stack of jewelry on the counter, or that valuable watch on their wrist. Again they are going off historical trend and it isn't to say all bad credit people are bad, only that when people do bad things, they tend to have financial problems so it is a risk worth avoiding.
Generally speaking though, all these institutions have "forgiveness" policies. For example, the government background check might say your debt was back when you were a college student, over-extended a bit, and you attempted to pay it off once you got a job and haven't made any bad choices since. If the bad credit report was recent, then it might be harder to write off as they can't tell if you'll indeed pay it back and it might mean you have a problem that might not get resolved.
It all goes into age of debt, the amount of debt, and whether or not you attempted to fix it. If you did none of that, chances are you won't get hired by these types of jobs.