We have done this twice. Once worked well, once was horrible.
The problems are -
1. They continue to treat/think of it as their own place. They don't necessarily have a "tenant" mentality about things like what they can do, what they can change, how they take care of it, and they don't necessarily understand their responsibilities as a tenant because they haven't been a tenant in quite some time. Our relatively good tenant was under the impression that she could break her lease at no penalty when they found a house - she misunderstood the agent's statement that it was a good rental market and they would probably find a replacement tenant quickly to mean she had no risk at all. She hadn't rented in 10+ years and just didn't have a clue. It all worked out okay, but she was really upset with the agent because she felt she hadn't fully understood what she was getting into.
2. Unless you do a condition report on the day of settlement, you can't really prove whether or not damage pre-dates the settlement. (This came into play when our tenants kicked a hole in the wall. Fortunately they didn't fight for their bond).
3. They aren't necessarily vetted in the same way. You can't get former landlord references if they've owner-occupied for a decade. We assumed that, having serviced a mortgage and just received $200K+, our bad tenant would be capable of making regular payments on time, but once they were renting they were very slipshod.
4. You have to really monitor your property manager. I think PMs are a bit laid back about these arrangements - the tenancy is sort of a fait accompli. Our PM at the time (admittedly a bad one, which is why he isn't our PM anymore) didn't make sure the lease was signed before settlement. So for two weeks, we had these people living in our house without a legal right to be there, and we had to offer some things we didn't really want to offer (like an escape clause) in order to get them to sign one and get some legal protections. The PM said that "it was all under control" and "they were coming in to sign" the day before settlement and didn't tell us they didn't show up.
I don't think we would do it again. Personally, I'd say just delay settlement until they find a house. Look at it this way - you're getting the house at a few months ago's price without making repayments.