Acquihires doesn't have necessarily the sole purpose of hiring new employees. Take a look at Facebook buying Face.com, Face did create a technology that was interesting for Facebook, it wasn't just the employees in which Facebook had an interest on. The tech now belongs to them, and they can use it to improve their own, or even replace it. Better than that, the original Face team can now work with the Facebook's team that was working on face recognition software, cooking up new and better ways of doing that. Of course there are risks here of the teams not meshing together well, but this is all analyzed and decided upon before the acquihire.

Other than that, acquihired employees aren't necessarily interested in being hired the other way. When inside a company, you have to climb the hierarchy ladder, you won't be hired after college to just be the leader of a big team, or the CEO of the company. For the people not interested in climbing the ladder, building a startup and being acquihired is a very good way of hacking the ladder and landing right on top, or close to it. I recommend pg's essay on this ( http://www.paulgraham.com/hiring.html ) This, together with the fact you have already mentioned that these acquihired employees have proven the ability to create something, reduce the employer's risk on selecting the wrong person to do the job, making acquihiring a good investment opportunity.