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.