(this is all said with respect to an engineering position)
In an early-stage startup, you are likely to be building the product and doing at least as much useful technical work as the tech cofounder from whom you are taking some of the burden.
Moreover, because you're in a small company, you probably don't get benefits, and you probably don't get good pay ("After the next round!"), and you probably don't get much gear, and so on and so forth. The options you're granted may well not pan out, and even if they do they probably will be diluted into something equal to a holiday bonus.
That being the case, it makes great sense to look after your own interests.
This equation, of course, changes once the company is large enough to actually take proper care of its employees and pay proper market rates.
Yes, it is the difference between being a parent and being a babysitter.
Both are responsible for the health and welfare of one or more children, but the babysitter eventually gets to go home and be "off the clock".
Problem is, many small companies seem to think that the equivalent of calling up the babysitter at 2am to deal with a vomiting child is acceptable.
I'd take the extra $10k a year over stock 95% of the time, thanks. Or how about a matching 401(k)? Those are the kinds of compensation that show consistent returns right away, and aren't a roll of the (rigged) dice like employee equity.
Assuming success at a $50 million valuation, you did all that sweat equity for 25K, while probably giving up a lot more in salary.