Take one of the obvious benefits, for instance: Health insurance. There's a big difference between the cost of insurance you get at a big company (one employer covered _two_ sets of glasses per person per year!) and what comes with the health benefits from a smaller firm (my spouse's company has a reasonably affordable plan, but we have a $12,000 deductable... which we hope to never need to use up).

There are other tradeoffs, of course, and he loves his job so he isn't looking, but the point is that you don't know what the benefits are, going in. They are absolutely part of the compensation package a developer (or anyone else) has to consider, but nobody willingly offers the details until the job offer is on the table, or until Day 1 on the new job.

Some of these things are negotiable, so I can understand why they rarely are mentioned. For instance I had some CIOs recommend that in salary negotiations, if the company balks on more money you can ask for a third week of vacation and usually get a Yes.

But yeah, I'd like it if Glassdoor collected such data!