The XP gain is there to quicken the pace of the match. Since everything else has been increased you have to normalize that with an XP increase or you're going to run into a few issues. Like some of the champions being under leveled but they can still afford Infinity Edge. The skill cap doesn't decrease by a low margin at all considering CS is not an issue in dominion. In SR it would be a huge skill cap issue considering that regardless of whether you "win a lane" or not your opponent would be the same level and roughly the same gold.

In Dominion though, it's different. You're not in a "Laning phase" against your opponents to further your teams progression. Nor do you need CS to increase your gold gain over your opponents. Tactics in Dominion is where the real skill will be tested. Can teams work together? Are you paying attention to your minimap? Are you controlling the buffs? Are you denying the opponents buffs?

The skill in Dominion will have a different element, it will rely on your team fighting and tactical decisions that are made quickly that will have an impact rather than your CS and lane management.

The xp gain doesn't hurt anything, and how does it lower the skill cap when everyone's getting it? you're all on an even playing field.

Even though you do gain passive exp, I'm pretty sure you'll still gain more exp from champion/minion kills, albeit perhaps to lessened effect.

Plus, if you're just sitting there waiting for exp, you're not actively capturing points or killing enemies, and thus being a handicap to your team.

sounds like a feeder 2 me