I appreciate you Samurai but I believe that you often get your hackles up in optimization discussions because you on some level feel like your enjoyment of the hobby or how you like to play is being threatened.
Certainly, I think you agree that Infiltration Expertise is weaker than normal skill checks in at least two respects:
- you can't impersonate someone
- you need a week of prep.
Both of these shortcomings are worth considering. Of course as I said earlier it has advantages as well. The biggest by far is if you're in a deep cover context and we assume that infiltration expertise covers bluff checks. Having an unfailing bluff/disguise/forgery against anyone you meet is pretty useful in such a context!
Personally though I find that kind of deep cover scenario where you have a week to prep ahead of time to be something that would simply never happen at any table I've played at, so the upside is barely relevant. It's always hard to establish what the heck most people are doing in their DND games, but I feel pretty confident that this is rare overall, and the biggest issue is that this creates a dilemna for the table since you're basically doing solo play.
Why can't you just take people at their word? People don't call it useless, and there's no 'dog-whistles' here. We're just stating opinions on DND lol. If we truly thought this was worthless we could just say so. For example: the grappler feat is worthless. No need to mask my opinion there.
But by your own admission its not good in a typical D&D game, a game where people are adventuring and doing combat (DND is an adventure/combat system) which kind of just means its not.... that good. We can posit about some hypothetical game where this ability is useful, but its all going to be hypothetical. I really have no idea what an 'infiltration heavy' campaign looks like. DND doesn't really support that, and most of the stuff in DND that's relevant to this is, well. Divination Magic, which is allowed to be hugely overpowered for anti-iniltration purposes because the game doesn't really care about infiltration.
It's like trying to run a classic Poirot mystery in DND. Paladin casts Zone of Truth and the whole thing is over in 10 minutes. The game isn't set up to mechanically resolve a complex series of clues and hints. Mechanically, the game is about exploring and fighting and hanging out with your friends. Those are the things that the game tries to balance.