They have their place, just like 2 3/4 and 3" shells. I shoot mostly 3", but there are times when a 3.5" shell has a distinct advantage. I generaly don't suggest everyone shoot them, because an appropriate 3" shell will bring down any bird if choked and shot properly, and because a lot of shooters already have bad habits that a heavier recoil load will only magnify. In the right gun/choke, and the right shooter's hands a 3.5" shell can be a marvelous thing. I shoot a 1 1/2 oz 3.5" 12 load that nearly half the time I drop 2 birds with (46% currently). I don't shoot it as much anymore, but I load it for some friends who are big guys, have decent form, yet still don't shoot very well. Gives them a nice wide kiling pattern. It's undeniable that those extra pellets CAN be of benefit. Whether they ARE of benefit has more to do with the the shell, the choke, the gun, and the individual, than the fact that it's a bigger load.
2009/10 Season Totals:
and my very first deer, should have done it decades ago