To be fair, the extra Personas don't "count" as part of your Compendium. You can still summon them and fuse them and all, but you won't need them for the trophy to pop or anything. Most of them are noteworthy Personas from earlier games that would mean something personal to the people who played those previous games.

The DLC costumes also contain special accessories that you can't get anywhere else. I didn't know if that was mentioned yet, but it's worth bringing up.

Is it worth 5 or 6 bucks apiece? Probably not. But you're not missing any technical "game content" by not purchasing them. Events will still play out as before, and heck half the time your costumes won't even show up (cutscenes and the like).

Also, I think the idea that most people are only going to purchase one or two costume packs holds more water than it seems. Do you really think anyone feels compelled to buy every single available Persona 5 theme and avatar set in the store?

No one's holding an Evoker to your head to force you to buy DLC. If you think the price is outrageous, don't buy it or wait for a sale.

And you can't make a blanket statement to try to cover your ass by saying "all companies that do this make me angry" when you came to this specific subreddit to make this specific complaint against this specific DLC offered by this specific company.