Playing the averages, sure, a boat kept out of the water "should" fare better over the years than one that is kept in the water. BUT, that absolutely does NOT mean that a wet slipped boat (and there are many, MANY of these throughout the States... and of course the world) will be bad. There are WAY too many factors and variables at play to make a blanket statement. As long as the buyer does his/her due diligence during the "checking out" phase, I personally see nothing wrong with a bottom painted boat.
A lot of this boils down to how someone plans on using their new boat. If they're going to keep it on a trailer or a lift, then sure, a clean bottom makes more sense. On the other hand, if they're going to slip it (or the aesthetics of a painted hull doesn't bother them), then purchasing a boat already painted can be seen as an advantage.
It's all personal preference - there will never be a "best" way... just like there is no "best oil to use"