BeauMan,
In my experience, and that of others I've talked to, an interesting thing happens the longer you fly and the more types you fly. You start to become more experienced at switching between types. I notice it particularly, flying both f/w and helicopters. In the early days, it caused confusion. These days, recent practice on one seems to improve my performance on the other. After all, to a certain extent flying is flying, and many things are similar, no matter what you fly.
To take your example, I'd say it depends on that particular PPL. If he/she is fairly new to flying, then two types may cause confusion. If the said pilot has been flying many types for many years/flying hours, it probably won't make a great deal of difference. It may even help, because if you fly different types, you are aware of the problems - you look at a control before you move it, you study the important numbers and if necessary write them down, you take a huge amount of care because you understand the problems involved.
But like I've said, this varies between individuals. It's why I think that ANY hours requirement is useless and in fact counter-productive. Since to many people legal means safe, it may give them the impression they've flown enough, when they haven't. And conversely, the legal minimum may be quite unnecessary for some people.
Pilots are trained to make decisions, about when to fly, where to fly, how to fly etc etc etc. Doesn't it make more sense to let them decide on their own ability and currency too? Why are we treating them as adults most of the time, and children in this one aspect?