The shortsightedness of your comment amuses me.
Even though you will seldom (if ever) touch the back of your iMac, the fact that they are using desktop components that generate so much heat for such a thin thermal envelope means that they need much more in the way of sensors and fans to keep it from overheating. I'm sure you won't open your iMac. Most won't. Most can't. But when your hard drive inevitably dies, you'll need a special hard drive from Apple (read: no third-party drive) that has the special firmware to turn the power pin that would've been the hard drive indicator light (which you don't find on Macs but do on PC desktops) into the connector for the drive's internal temperature sensor, WHICH YOU NEED BECAUSE THAT iMAC GETS THAT HOT, and it will cost you a stupid amount of money when your AppleCare dies as opposed to the third-party drive, which is an option in literally every other Mac because those Macs don't get so close to overheating. But hey, you'll never have to open it up, so why worry?
Also, increased heating issues makes for decreased reliability. At my work, we had more iMacs than any other computer as far as non-accidental/non-REP repairs were concerned.