You and your clients are presumably not peers. While it might make sense for some of your employees to jump to or from them, it's not nearly the same as jumping to a competitor of yours.
Of course, if you aren't in California you can lock them down with a non-compete, but then again that's why (I think) Silicon Valley has been so consistently and overwhelmingly successful (it's the one absolutely unique advantage it has).
This is something I've been harping on about lately but no-one seems to listen. Silicon Valley is the only place I know of where it is illegal for a company to restrict the work you do after hours and without company resources. This is such a massive boon for people trying to create their own startup. It means you can work on a project on the side until you are ready to commit without having to worry about your current employer stealing your IP.
No one listens because it only directly helps competitors, and of course startups by definition are not entrenched businesses.
Few are wise enough to realize the advantages of an overall healthy ecosystem, and the giants like Intel, once they became giant, only want that to a limited extent anyway.
Actually, they sometimes are. We solve hard consulting problems for competitors quite frequently. And those are then the cases with most need for an agreement like this...
(By the by, I'm a fan of the laws allowing people to build their own stuff in their spare time without using company resources).
Of course, if you aren't in California you can lock them down with a non-compete, but then again that's why (I think) Silicon Valley has been so consistently and overwhelmingly successful (it's the one absolutely unique advantage it has).