I'm no expert, but wouldn't it be a semi-bad idea (from a consumer standpoint) to make most of your heat dissipate through the most important part of single-piece product?
Although not extensively, that sounds like it would significantly shorten the life span of the product. Not the internal parts that cause the gadget to function, maybe, but the only actual part that the user ever interacts with...
Of course, it's very likely that I'm wrong and there could be no problem whatsoever, but one of the basic things I know about computer parts, is that higher heat in a part means several different things that all end up pointing in the direction of a shorter lifespan on the specific part, and I don't know why this would be any different.
The MacBook Pros are milled from a solid hunk of aluminum. The heat is vented along a 10" or so long section where the screen hinge is (the hinge is very long). The heat is sufficiently dispersed that it's not hot, just a little warm, and the product is really a very impressive hunk of engineering. Remember that the way that Macs are designed, there are far fewer independent and moving parts than in most laptops, which means there's a lot less to wear down, misalign, etc. That gives them quite a bit of room to engineering around a little added heat.