The item debate is an old one. Aren't you really just assuming that it hasn't been tested at length? A link was already produced that had extensive testing and debate with the aim of creating a standardized item rule set. It, obviously, didn't become mainstream.
In Melee, the number one most important argument was that there were explosive containers, which is enough to warrant a ban.
In Brawl, after extensive testing, the Item Standard has been established and to my understanding may continue to be a work in progress. Especially with the new games on the horizon.
Honestly, I'm neutral on items. Strategy, techniques, and skills have to be modified if they are introduced. Most of my experience has been with melee and I have some small experience with brawl. (I put a lot of debate into the original thread listed above all those years ago, as well.)
So, maybe someone with more experience with Brawl could provide some more meaningful input, but as far as I know, almost every item is not worth the trade off of losing your aerials. Nine times out of ten, the optimum choice is to throw the item.
Look at the legal items:
Most of them are activated by throwing. What's left over either consumables that aren't even necessarily worth picking up (Bunny Hood) or better off being thrown (Lip's Stick, Fire Flower). The only real advantage here is for characters who don't have projectiles to throw at characters who do have projectiles.
Another issue (again in melee, don't know about brawl for sure) was that it pretty much only reinforced the tiers. For example, the stronger characters had less "item pick up frames." The shortest, I think, was Fox with 1 or 2 frames. combined with his speed, and shine just make items great for him.
Sure, if items are turned on, you have to play differently. There were several times in the invitational where some of the players were not actively pursuing powerful items on the field and ended up paying dearly for it. Just because something is part or not part of the game shouldn't automatically affect its inclusion in competitive play.
Example: Tripping.
I know, I know. You'll say it isn't the same, but it is. Tripping was a random element sure. It could be avoided altogether though by moving predominantly through the air. Just like using items have risk/rewards, moving excessively on the ground did as well. It was included a mandatory included feature in the game. If you were "TRULY" good at the game, you should be able to win with tripping enabled.
As far as the claim that melee players have a harder time dealing with item characters, I would say that evidence is needed. Even if that were the case, it should only be at first because of item-specific special techniques and will eventually not be an issue as their experience within the game grows.