Other than the fact that compact gearing doesn't work for everybody. If I ran a compact and used the same equivalent gears for riding as I do now, I'd have to shift down to the 34T ring every time I stopped and for just about every time the road started to point up. With the 42T inner ring on my triple, I can handle all the stop and go no problem and only need the 30T inner ring for certain extra steep climbs. Trying to use a 50T ring for most of my riding would result in a lot of unnecessary straining or a lot of extra front shifting. Now, the latter isn't necessarily bad but when it comes with the tradeoff of reduced range (for a given cassette) and more double shifting due to the larger gaps between chainrings, I have plenty of reasons not to go with a compact. YMMV.