This generally follows what other targets do. I don't completely understand why the special case for tail calls existed in the first place; even when the code was committed in r105413, call lowering didn't work in the way described in the comments.
Stack protector lowering breaks if the register copies are not glued to a tail call: we have to insert the stack protector check before the tail call, and we choose the location based on the assumption that all physical register dependencies of a tail call are adjacent to the tail call. (See FindSplitPointForStackProtector.) This is sort of fragile, but I don't see any reason to break that assumption.
I'm guessing nobody has seen this before just because it's hard to convince the scheduler to actually schedule the code in a way that breaks; even without the glue, the only computation that could actually be scheduled after the register copies is the computation of the call address, and the scheduler usually prefers to schedule that before the copies anyway.