Because now people are carrying an MD5 implementation around with them, and when the next vaguely-hash-shaped problem comes up, they're gonna reach for that without really evaluating, b/c they've used it before so and it's at the top of their mental cache and it's even more convenient now. :/

I guess it's not entirely equivalent but I'm still sorta reeling from a "why should we introduce [something like bcrypt] for password storage when we already have these other perfectly fine hash functions in use in our codebase" convo at a previous project.

No No No. Then you add another dependency on MD5. And MD5 is a very slow, complex hash function. Use MurMur, City or any of a number of really fast, non secure hash functions instead.

A situation where it is defined not to matter is unrealistic. Things you don't think are "security critical" have a tendency to unexpectedly become so; at least that's my impression. Why not use a more secure algorithm?

Exactly this. I dont like when people try to sell new things even though my use case is perfectly fine with the older solution. I understand the security implications, yet there more to this topic than just security.