Possibly, yes, I mention that at the end of the bit you quote, and also in other parts of my post you chose not to quote. It would also depend on the system in question, and so on.
However, you brought up the example of the fine attire as something beneficial on which a PC can spend gold. My point is about how having a concrete benefit ahead of time that says "+2 on influence rolls" or "this roll is made with Advantage" or "the NPC's starting attitude will shift to Favorable" or something similar can actually serve as a limitation on play. When there are existing mechanics of this kind, players tend to say "Okay, we need to influence the noble...what can we do?" and they consult a list of actions the rules already addresses, and they limit their decision to those options.
Those mechanics codify what happens when the PC wears the attire. The GM does not need to determine what happens. Would you agree?
If not, and you feel the GM does have a strong role in the outcome, then I'm not sure if we're disagreeing. Perhaps it's just where the GM judgment comes into play? For you, it's okay for him to determine the DC and the results of a success or failure....but if I understand correctly, not in what's possible?
For me, I prefer if my players come up with what their characters do, and not decide what's available to them from a predetermined list, and then as DM, I can decide if a check is needed, and if so what kind and at what DC, and then the results the check.
You can list attire, or not, just as you can list weapons and armour, or not.
D&D lists weapons and armour. I don't believe it even mentions attire in this context.
Prince Valiant doen't list weapons at all (or rather, it indicates that a decent weapon grants a 1-die bonus to the pool) and it lists armour only as light, medium and heavy. It also mentions that attire and other signs of status or prestige can give a bonus when trying to influence others based on that status or prestige.
Burning Wheel has very detailed rules for weapons and armour (comparable to RuneQuest or Rolemaster in compelxity). It has sparser rules for attire, having "costs" (which in BW take the form of "points" in PC creation and difficulties for Resource checks in play) for normal gear and (what it calls) "finery", plus general rules for advantages and disadvantages on checks, and mentions that what one is wearing might confer an advantage or disadvantage on a check.
Marvel Heroic RP doesn't (as best I recall) mention attire, but it has a generic rule for creating resources. I once had a PC use this rule to equip Nightcrawler with an image inducer to get a bonus die when meeting people at a nightclub. Attire might be established via the same mechanic.
I'm increasingly unfond of static bonuses on roll-to-beat-difficulty checks, because they muck up the maths. So of your possibilities I would favour advantage over a +2. This is closer to introducing another die into the pool in the other systems I've mentioned.
I don't think that suggestions (for either players or GMs) about ways in which advantage might be gained, and suggestions about how this might correlate to money spent, are limiting. That's certainly not been my experience in systems that have them. I especially think that guidelines that help GMs manage the maths of what are rather intricate systems are helpful. To pick just two examples: Rolemaster is pretty hopeless at this; 4e is pretty strong at this. And when the GM is supported by robust guidelines over the top of robust maths, I think it's easier to run with player ideas without worrying about them breaking the system.
(Which presumably is one worry, maybe a major worry, for those in this thread who have expressed concerns about powergaming.)
