Also, in regards to the magic item issue, I don't think you're understanding the point.
A level 30 character might have something like a +32. That's +15 for level, +8 for his main stat (started at 18 and pumped it every level), +6 weapon and a +3 for weapon proficiency. Another character in the party might only be using a +2 proficiency weapon, and he only started with a 16 in his main stat, so he would have a +30. That's a difference of only 6.25%, which is fairly minor.
Remove the level mechanic and you will instead have a character with a +17 and another with +15. That's a difference of 11.76%, which is a little bigger. This is what the others were talking about...without the 1/2 level mods you lose the padding. Suddenly, a character's weapon becomes a LOT more important. My Axe and Shield Fighter is suddenly missing 5% more than a Fullblade Fighter, or perhaps even as much as 10 or 15% more if he had a slightly lower ability score, or had an effect like a mark that was giving him a negative to hit.