Nothing. It is quite common to do it like this here in the Netherlands. Of course, once we have a new employee and after a couple of months we think she is a great addition to the team, we will start negotiate for the permanent contract immediately.

Our team consists of experienced and above-average educated developers. We are not looking to attract that one 10x developer, nor do we desperately need a senior developer now. We sort of have too many senior level developers; our retention is high. Anyway, we are looking for similarly educated and experienced people that fit our team, and we offer a culture where such a person can thrive. We prefer to take a while longer to find and grow a great match rather than to gamble on potential talent that might leave again soon.

On the other hand, we do need new developers, particularly specialists, and we have gotten to the point that we are willing to take more risk. But the organization as a whole is still figuring out how to grow beyond a 10 person post-startup development team towards a 100+ person company (we merged with our biggest partner).