it's not exactly scientific reasoning, but when I was looking at boats I automatically excluded anything with a GRP top, as mixing the materials just seemed to me to be introducing potential problems that didn't have to be there.
The obvious issue is different rates of expansion between the steel and grp potentially causing leaks in future, plus if you happen to run the front end of the cabin roof into a bridge or suchlike, I know which will suffer more damage!
It's personal preference really, i'm not aware of any major issues as such, but probably less desireable than all steel so may be harder to sell later, but if they like it and the price is right.......
Peter