Now that the enforcer's been nerfed to the point where you can't just bullshit your way out of direct combat with it as spy anymore, it's no real surprise that the amby is making a comeback. There's a reason why "pseudo-sniper" is considered one of the highest-end loadouts for spies, as it's a low-risk vs. a potential extremely high reward if you can just keep your aim straight.
Amby, Dead Ringer, and Spycicle. One weapon to do all the killing and the rest to serve as buffers against anyone who gets pissed off enough to come after you, if they even survive that long against consistent 102 damage shots. There's a reason that a good "pseudo-sniper" spy is virtually impossible to kill, because he's usually just sitting back killing everyone that gets even remotely close and then popping his get-out-of-jail-free cards every time someone manages to close the gap. Soon as they give up, he reappears someplace else and the sniping begins anew.
The fact that that amby's accuracy resets faster than a skilled sniper can get off two consecutive quickscopes, and can be done with much less pressure since you don't have to look through a scope to get your extra damage, basically turns spy into a much deadlier sort of battle sniper. Hell, even though the amby doesn't do 150 damage on an uncharged headshot like the sniper rifle, two 102 damage shots is still enough to take down a full-health soldier and anything below him in the HP department. The only thing pseudo-sniper spies give up in relation to the actual sniper class is that they can't two-shot quickscope full-health heavies, requiring three shots instead, and classes that would normally take one sniper headshot take two hits from the amby instead, although the amby headshot would reduce them to a low enough health that the spy would only have to bodyshot them afterwards to finish them off.
It's true that the amby can be somewhat difficult to use if you can't aim, but the fact is that pro spies who know what they're doing become an absolute terror with it equipped, and also become nigh unkillable due to spy's typical evasion tactics.
They're literally quickscope snipers who give up some of their damage potential for vastly increased survivability and far less pressure when aiming their primary. And even if the amby fails them, they always have the usual spy tactics to fall back on with the knife and it's all too questionable hitboxes.
TL;DR: Amby spies do insane damage from a safe distance, while being insanely hard to kill at the same time. Damage falloff could potentially solve this, while keeping the amby relevant for dealing with razorback snipers.