Again, if that works for you and your table, great, go with it, and I hope you have a great time playing with it, but I would not recommend it as a general approach for all tables, and feel that Narrative Control as-written is well balanced (and is of course completely optional). A great strength of the system is how much tolerance there is for a given group to tweak things to their satisfaction.
On top of that, SPENDING the die also makes the luckslinger even more obscene than simply rolling one, as they can get upwards of 10+ action die to throw around as seen fit. If they have to roll one there's actually some chance involved, they can't just shovel four of their bonus d4s at you and say "We win."
Honestly, I feel this is a non-problem
1) GM always has final say on if a Perk is allowed or not, 2) There's an upper limit to how good a Perk can be and how long it'll last. 4AD might get you an 80xp ally for a single scene, or fast forward through an instance where the action has slowed down, people are frustrated, and the fun has stalled, but it's not going to auto-defeat the final boss for you. The 4AD spent to influence a ruler to act in your favor this time has no bearing on next time (rulers are notoriously fickle like that).
3) If someone decides to blow their entire reserve on a single moment, think of all the opportunities they've passed up to activate criticals, opponent's errors, roll boosts, etc. Remember, if you've got a boatload of AD to start the game, so does the GM. A phrase from Spycraft comes to mind -- you control the vertical.
As a GM, if your players are being miserly with AD and hording them up to the end of each session, this sounds like permission to take off the kid gloves and get to whomping on them much earlier. Similarly, if they're hesitant to spend AD, perhaps it's because not many are being awarded in-game.
Additionally, as a player, if my group scrounges up and spends 4 AD (let's say, for the sake of argument, all of them came as rewards for good roleplaying) to ask for a Perk, only to come up 1 or 2 points short of the required number, I'm going to feel a little frustrated, maybe even cheated out of a valuable resource.