I'm glad this question was asked. It's been on my mind a lot lately and I have some pretty strong feelings about it.
I've long maintained that it's as easy to be underbowed as it is overbowed... with similar negative results.
I agree with Fred Asbell per Gregg's post. If your muscles aren't working to shoot the bow it's far easier to do all kinds of weird things before the arrow clears the bow.
Recently because of "perceived" aging I'd gone down in draw weight considerably. The accuracy that I once had suffered and I wrote it off to that same aging thing. I'd just have to accept it... but I couldn't and wouldn't be satified with less than the best I could do and I knew it had to be a lot better than what was going on.
Not having had a new bow in almost ten years I decided it was time for me to have one and I finished it a few weeks ago.
I brought it to weight 5 pounds heavier than I'd been shooting and the result was an immediate improvement in my shooting.
Once again I can draw up on a target, anchor and hold and KNOW the arrow is going straight away to the target.
It's not just about getting a better release with the heavier weight... my release is just fine with lighter weight.
It's my opinion that many of todays archers are much too relaxed at full draw with their lighter bows.
And shooting a weight you can shoot all day is a matter of conditioning... really, if you shoot a 3D course how often do you shoot anyway.
If you were standing in one spot shooting a dozen arrows at a time I could see being fatigued at some point.
But shoot an arrow and waiting 5-10 or even 15 minutes between shots allows plenty of time for muscles to rest.
In my life I've shot bows in the upper 80# range and snagged more than a few tournament wins with that weight.
I had progressed down the ladder to 62# and now am back to shooting 67# at the age of 61 and I'm tickled pink with my shooting again.
There is no excuse for shooting any weight bow with bad form. If you can't reach full extention with the weight you are shooting then either improve your shooting strength or admit you are overbowed (no shame in that!)
Finding the "optimum" draw weight for your body type and physical limits is a worthy quest and can be difficult to find.
Ideally it's not one more pound than you can shoot accurately or one pound less.
I sure hope this doesn't turn into a heavier/lighter debate. It should be about what is best for YOUR shooting.