Eh, I think the conversation has drifted into technical semantic crap.
One Side: "When I DM, the players decide stuff. Because, you know, I let them."
Other Side: "But YOU LET them! That means really you decide stuff!"
One Side: "Right, but I decide to let THEM decide..."
Other Side: "Right, but YOU decide to LET them decide..."
Blah blah, implicit gaming contract, mutual consent, blah blah. In a good game the players have some feeling of control and also some feeling of uncertainty, and the DM gives them some things they expect and some surprises.
- - -
Anyway, back on track. IMHO it's far too easy to build a one-trick PC. This is terrible because when your trick works, you win, and when it doesn't, you lose. Diplomacy builds are particularly terrible in this regard, because they win by talking and not killing stuff.
This is what silo'd abilities are intended to solve, I think.
Cheers, -- N