I am going to base my argument off of my experience with Brawl, for what it's worth, because apparently the skeptics only take that as evidence. I am by no means a pro. But I hope you will understand how I and many opposed to items actually feel, when we're playing the game, when items are on.
To put it most simply, when they're on I feel like they get in the way of fun. ASIDE from some being overpowered (e.g. spicy curry), ASIDE from lightning or clocks spawning right on top of people and giving them an easy advantage, having items spawning in the middle of the match just FEELS wrong. If we're both standing on the stage and an item spawns on him, and he grabs it, SURE if I had skill I could probably dodge it and deal with. But items generate the same discontinuity of play as tripping does, even if the lowly fireflower doesn't threaten to kill you outright. The match is flowing one way and suddenly a hill rises up out of nowhere and everything is redirected because of random chance. It just feels WRONG.
I've played SF third strike (not competitively, but still), and I get good feelings from it when I do things right, when I predict how my opponent will attack after blocking his aerial kick, then blocking his standing and crouch attacks, or land a good hit with some predictive capability. It has flow. I'm not good at it, but it has a good flow.
I get a similar feeling in Brawl and Melee with items off. I'm better at Brawl, but when things happen they happen as a little dance that I and my opponent do. Except for tripping (WHY GOD, WHY), the game doesn't interrupt our dance at all, and leaves us to do it until one is finally KOed. In short, items are like having clippy interrupt you every 30 seconds as you're writing a term paper, or finding that your legs change length while you're running a mile. They just ruin your Zen.
Smash balls are slightly different, but the effect is similar, though other arguments can be made against them.
For one, they aren't at all balanced from one character to another. Falco's landmaster can take off several stock easily off one person. Compare that with Lucario's, which nearly any character in their right mind can just JUMP OVER until it's done. Ness's and Lucas's are aimed at chaotic FFAs but 1vs1 are actually easy to dodge. Metaknight's FS sucks. Marth's is awesome by comparison (though falco's is still best since it can take off multiple stock). Kirby (who is very similar in air mobility to Metaknight) has an absolutely sizzling smash. And none of this has any relation to how good characters are without FSes, so it makes otherwise fun and balanced characters worse. Brawl is more balanced than melee... UNTIL you add FSs.
You do have to fight off your opponent a bit, but I just don't get any feeling of triumph when I kill someone with a final smash. You push B at the right time and they die, or if it's more complicated than that you have to push some other buttons to kill them as with the landmasters or Giga Bowser. Sometimes FSs pop up so much the only kills you ever get are from FSs and you hardly do any fighting without one person or the other using them.
Not to mention that the more of other items you disable, the more FSs pop up. I hear the lowest you can go with just FSs (no other items) is a thirty second interval between each (since you can't get two FSes on the same screen at once). At that rate it would be surprising if you KOed your opponent any other way!
And in the end, this just comes back to my feeling argument. You get in the zone with yourself and your opponent, and then a smash ball pops up and suddenly everything is changed. You can chase the smash ball, you can chase the chaser, you can call it strategy. Heck, that's what it is. But suddenly, in a situation where it was just you and your opponent, this item, this third party comes into play, and if you're LUCKY stands between the two of you and says "Come and get me".
It's a completely different game from the one I enjoy playing, even with my pretty scrubby friends. Smash with items on is a game I will *never* play competitively. I may not be able to prove to you that it's not a good idea, but I, as a hopeful competitive smash player, would rather dance without interruptions.
my 2 cents