Yes, and your computer gets turned into a space heater, draws an extra 120w when it's idling - 5850 only draws 25w idle - and you get all the lovely issues with games that don't support Crossfire properly [or at all] having bugs and glitches.
I have no idea why you are evangalising the 4870x2, it's end of life, and now officially obselete seeing as the 5870 does almost everything it does with none of the fuss and having a nice, futureproof DX11 chipset, and 5850 appears to be not far behind it at all in terms of raw pace.
Admittadly, given the speculative performance of the 5850, I'd give the 5870 a miss for the moment myself, but I wouldn't turn it down for a 4870 just for a few FPS, especially considering the other, aforementioned downsides the 4870 now has in comparison to the 58xx series.