I mean, if you really want to get into it, it was a huge pain in the ass at a time when paying the cost of a call to islower_english was much more expensive tha

I mean, if you really want to get into it, it was a huge pain in the ass at a time when paying the cost of a call to islower_english was much more expensive than a hardware less-than instruction. We've broadly moved beyond that, but there's still value in grouping sets together in a way that makes certain kinds of frequent tests less computationally expensive than they would be if codepoints were randomly distributed.