I think it is something I think all employers would do well to keep in mind. But especially startups, because as you said, you are almost always getting paid below market rates. And, on top of that, it's a rather unstable job; I mean, if the company goes under, you get fired, right?
The thing is, when you are a startup, you /don't need/ people who want to stick around for 5 years. In 5 years, you will be much bigger, or dead. Either way, the employees you need 5 years from now will be /completely different/ from the employees you need now.
As a startup, you want ambitious people; the sort who hop from job to job to increase their salary and to get more interesting projects. You want someone who is relatively bright but inexperienced who really wants to gain experience. There are people who require constant change, and those people do really, really well at startups, and not so well at large corporations. Often, those people are willing to work for less money in exchange for letting them work in things they are not qualified to work on, and nearly always those people are willing to trade away stability, which is important, because as a startup, you can't offer stability.