Yep, that would be an interesting new product and I guess there are people who want it to happen. But it would be a completely new product for Apple, since they never build anything like it before? Is it worth it for them? Most likely no. These kind of computers are very niche and usually don't sell well.
Personally, I am conflicted about a "MacBook Pro Pro". On a plus side, it would generate positive publicity and shut up some of the circuits, even if Apple ends up losing money. On the minus side though, I am worried that such a laptop could let Apple to relax its demands on the current 15". What makes the 15" an excellent computer in my eyes is that its very compact and super portable, but still offers plenty of performance. If they were to downgrade it "because we now have a desktop replacement for those who want performance", I would be very disappointed.