Smoke and mirrors. Wes Welker's strength as a receiver is getting open about 6 yards off the ball when the line of scrimmage is between the 20 yard lines (ie. not the red zone). Only 7 of his 67 catches came in the red zone for a reason.
It's easy to get open from the slot 6 yards off the ball, McMichael was able to do that too but he didn't have as good hands as Welker. What is a little more difficult is getting that reliable 4 yards of YAC that Wes is very good at getting. Why is he good at that? He's a quick thinker, a punt returner. He catches the ball and figures out where to go and who to juke in an instant. BUT, unfortunately, he doesn't have the speed to complement that quick thinking...and so the reliable 4 yards of YAC stays a reliable 4 yards of YAC, and there is no real threat of Wes breaking anything for 40 yards and a score. There's a reason this guy has only 1 touchdown to his name. There's a reason that of the 100 times Welker got targeted by our quarterbacks in 2006, only 11 of those targets came in the red zone. It's easy to get open 6 yards off the ball when the LOS is between the 20's. When you're in the red zone, spaces become tighter...and it's harder for a tiny, marginal athlete like Wes to get open.
Feeding Wes Welker the ball in order to exploit what he is good at can lead to offensive ineptitude. There is a reason that Wes Welker's breakout season coincided with the worst offense seen in Miami since 1967. When you are between the 20 yard lines, you need to be aiming to gain yardage in chunks. You need to be thinking fifties and hundreds, not nickels and dimes.
The more catches Welker gets, the worse it will be for your passing offense. That is a truth that Miami coaches were too damned stubborn to realize. They should have learned their lesson in 2005. During the final 6 games they moves Chambers into the slot and stopped throwing the ball to Welker so much. Chambers' production doubled, Welker's production halved, and Miami started scoring a lot more points, winning more games, and gaining more yardage.
Wes Welker's a nice luxury to have. But if he's catching any more than about 30 balls in your offense, then your passing offense has issues with myopia...and THAT is the cold, hard truth.