However, regardless if the laptop is Apple or Dell or HP, their heatsinks sit on the enclosure of the system. Therefore, plastic does not absorb heat like metal does. This is where an aluminum body helps a lot. The entire bottom part of the laptop becomes one big heatsink. Now you can remove the heat via the fans inside the laptop and the ambient air.
In a plastic enclosure, all the heat is trapped within that little heatsink. If, for whatever reason, the CPU/GPU produces more heat then what the heatsink can handle, then the system will fry.
If you looked at the back of the MBP, the entire area is one big vent. Only the two corners (for the 15") are the exhaust. The middle acts like a large intake. Not to mention for the retina models, there's side vents now too that draws air over the system board. On PCs I've owned before, they depend on little vent openings throughout the laptop or bottom for a type of intake.
I run my system hot all the time (for days at times too without rest and sometimes even on my bed) and I have yet to have any issues hardware wise. Remember, there's a lot more users in the world then there are forum members. Somehow, we think that one person or group who posts a failure of their system means a majority when really they're a very small minority.