Pressing for certification is double-edged, as many have presented here. I am astounded at the frequent lack of response here which misses the ideal way to keep such candidates, though. Simple, quality leadership is what keeps most in their duties (aside from love of the work). Its not usually about money, its about who you work for and with. If certs are suddenly compulsory and management isn't paying then its my view that they're probably not quality management to start with. That's what causes the high turnover, not necessarily the credential itself. Contracts are a complex, poor way of substituting for providing a quality work environment.
Yes, I've seen people leave once they got some cert or some specialized experience that is in high market demand. Guess what? They often tried to return and depending on how badly they burned their bridge on the way out, they might or might not have a shot at getting back in.
Incidentally, as has been covered here at length, management who thinks certs create wonderful workers are delusional. A cert is usually just a recognition the person understands the best practice being pushed by vendor X. Actual practice is proven in the proverbial pudding and the onus is on management to set expectations those best practices are followed.