I would mirror the other views above in general, except to add that the question of whether it should be allowed in a specific instance should really depend on party dynamics.

It's OK to give (for example) an unoptimized L6 Thief PC shield proficiency if the rest of the party consists of a GWM/PAM Barbarian, a Diviner wizard, and a minmaxed Hexblade/Sorcerer. At the end of the day, the only thing that everyone shares the spotlight, and no single character can solve every problem.