I've yet to ever have my 360 break down on me. You guys are probably just getting shit luck with the consoles they replace yours with, some are just made poorly I guess.
I had mine a year and a half before it shit out. And to someone above, they usually just replace the existing Xbox with a brand new one. So you don't have to worry about the permanent fix thing. They just go out so often it is hard to tell if it was a new one or the one you sent in previously.
Its just a bad design flaw. But my 360 could break down on me once every 6 months, and I would still play the hell out of it. Also, my 360 hasn't RRoD'd for 3 years... I play it probably 7+ hours a day.
They only replace a broken Xbox with a new one if you send it back 2+ times. The first RRoD, they replace the processor, which is the problem behind the RRoD (processor overheat).