While this is clearly the Right Thing, it is clumsy to have to
compose every constructor with transient->persistent, as you'll
have to do if you aren't intending to use any of the -! forms:
(cons 0 (transient->persistent (list 1 2)))
This kind of mandatory verbosity often triggers distaste and even
hostility from some programmers. It is especially onerous in the
context of implementations which don't provide any linear-update
variant procedures, where the distinction between transient and
persistent data is empty.
PS I would like to compare this again with the const qualifier of the C and C++ languages (with their respective semantics). There may be some programmers that are annoyed with compiler warnings when constness is not properly cast, but the majority isn't (I hope!) and instead makes sure that their programs are logically correct and make sure that every single cast makes sense. (And for the latter, it is important that the casts are explicit).
(NB: Of course, the const qualifier in C is not without problems, often due to historical baggage, but this is a question of good API design and not its general usefulness.)