Putting a user-facing restriction in place is very different from having your underlying infrastructure itself make that assumption. If the product spec says a user will be allowed 5 messages per day, and that's all you design the system to support, you will inevitably run into challenges when the number has to be changed to 20 or 2000 or 20 billion down the line. At that point "but we only used an 8-bit integer because you said the max would be..." isn't a good enough answer, and you are going to be stuck in a very messy rewrite.

Was it hurting anything to have millions of categories? I mean, I’d assume the metrics interface they’d gotten from you would be essentially useless, but if they had some workflow they worked, why not let them have it?