In certain cases, some "experts" believe longer crank arms (i.e., 180mm or longer) are more fitting for use by taller / long-legged riders. It helps them make better use of their leverage, so to speak. Of course, this also makes it harder for those riders to "spin" while climbing and what-not, and other disagree that there is much, if any advantage once you get over 175 to 177.5mm cranks.

If you're talking about a club racing or competitive scenario, I think any rider 6'3" or taller may be at a disadvantage due solely to power-to-weight ratio. Unless you can get your weight below 180 (see Hincapie, George), you are going to be at a serious disadvantage on anything involving heavy climbing or a headwind situation. In the sprints, however, riders with more leverage can kick some butt, if their gearing, cranks, etc. is set up properly and they have proper technique.

Of course, I personally just go out and ride to be riding. This is just some stuff I've read and/or observed over the years. I'm barely over 6', and I refuse to ride in any competitive situation - for now. I just like to see new scenery and go fast downhill - generally yelling "Wheeee!"