I couldn't find an answer, other than you're thinking it's unnecessary.
I summarized the points from #1233 again in the part you quoted there. But to repeat from that PR:
Also proxy plugins matching blindly against extensions, without checking the file contents, are basically broken already since all the plugins are only allowed in a single directory and the likelihood of extension clashes is great.
Further, proxy plugins that register more than one extension will most likely have to discriminate between extensions in their probe() function anyway.
But if you think it's useful to pre-filter by extension, then why limit it to caseless, single-dot extensions?
it just seems overkill at this point
Again, please describe what you mean by overkill (In what sense? "too flexible"? "too much code"? "too much CPU/memory overhead"? etc), it's not clear from the context what you mean. As I mentioned previously, it's actually less code, is no more complex, fixes the caselessness problem, supports multi-dot extensions, and allows various other simple patterns.