The first potential problem is that an employer-driven system can lead to workers being extremely vulnerable. They are here only for short periods and do not really understand the system, and so on. We would need quite extensive regulation to prevent potential abuse of those workers.
Secondly, if you are concerned about the social integration of migrants, it will not help with that. Inevitably, there is no point in people who are here only for a short period investing in building a life here, and links to the wider community.
Thirdly, historically it has been the case that, because it is quite artificial—at the end of 12 months a worker has to leave, perhaps to be replaced by another—it generally sets up some kind of pressure for employers to extend the 12 months. It may start off in that form, but there is a risk of drift into a more permanent migration route.