The problem is that prime (as it's used, anyway) doesn't carry any meaning the way ? and ! (as they are used) do in Scheme, it's just used for "I couldn't bother to think of another name". Enabling clearer names is great, but the language allowing primes in names was accomplishing the opposite of that.
I think that's a coding style decision, and shouldn't be part of language specification. I certainly don't feel about primes the way you do: having a function f and a function f' usually means they are sort of similar with some specific difference between them, definitely not "couldn't think of a better name". E.g., in haskell a prime often denotes a strict function, it's a better convention than having f and f_strict. Plus, how are you going to decide which unicode symbols should not be allowed? It can get a little random.
It's definitely a matter of taste, but I feel like settling for "similar except different somehow" is the same thing as "couldn't think of a better name". I would much prefer f-strict because then I don't need to be familiar with the punctuation conventions of the codebase to know how it's different from f, it's right there in English.
Edit: Plus, then you don't have to wonder if that particular function follows that naming convention; prime meaning strict isn't nearly as universally followed as ? or ! in Scheme, for instance.
