I agree with Bush although i'm sure it opens up a whole can of worms, will be shot down in flames and its a discussion thats been run many times before on here. I'm 6ft 3 and ride the 18" alpine. Our average day in the surrey hills tends to be riding up the fireroads, seat down at the top and then hoofing down the singletrack as fast as possible taking on the few in trail jumps/drops along the way. I think we're pretty quick on the ups, not race snake fast but a long way from granny ring spin all the way. I had a 20" 456 before the alpine and it felt like riding a gatepost on the way down, good bike but the size just wasn't for me.
I think it depends on where you live and the climbs/riding you do. If you're on for an all day epic in the lakes then i would agree, a 'correct' fitting bike would be better as the alpine would be uncomfortable. However for the short climbs we do i will happily accept a more cramped position to have the smaller, more maneuverable bike on the way down.