I have a less-pessimistic appraisal of OP_CAT. Yes, it enables more use-cases. Like anything, there are costs and benefits.

What I like about it, personally, is that the additional flexibility provides space for developers/users to defend against attacks. It may enable new and better CoinJoin solutions to mask transactions from censors. It could provide novel vault solutions. It opens up alternative signature schemes potentially with better properties for some kinds of attacks. Defense against a discovered weakness in ECDSA for example.

However, OP_CAT solutions tend to be costlier for the user than the would-be alternatives. An OP_CAT vault will take more bytes than an OP_VAULT TX for instance. But this is OK. Someone who wants the expensive functionality can pay for the bytes. If that type of transaction becomes popular, we can always fork in a specific OP later to reduce costs.

Having said all of that, I don’t think OP_CAT is something that should be rushed through. It’s not urgent. Consensus is paramount. I just don’t share the net-negative appraisal of most hardcore Bitcoiners here.