Skill or classed based is a huge decision. It"s one of the core design tennants and a major direction design heads once it"s decided upon.
The open ended-ness of skill based systems, imo, are incredibly tough to balance and require major development time above and beyond class based systems if PvP is to be part of a game.
I also don"t think you can ever truly "balance" an open ended skill based system, at least not during the early years of a games release.
Given the variables that millions of players introduce upon exposure to your system, and our desire to launch a game that"s as close to flawless as can be, it"s a tough call.