PBK serves no real function. So in a way it kinda of is like those other useless honor societies.
Only to some extent. In engineering, we have TBP (the second oldest/most prestigious honor society created because PBK refused to allow engineering students to compete with liberal arts majors for their pins) and I definitely popped that sucker off on my AMCAS.
After all, that's what all "awards/honors" are supposed to do: recognize you for something you have already done.
So of course it "serves no purpose" in that you are receiving the award for a past accomplishment. Hence the award, in and of itself, does not change who you are.
On the other hand, the reason why "honor societies" are often looked down upon in general compared to other scholarships/awards, is that many (PBK included depending on your school) are purely numbers based.
In reality, the numbers speak for themselves. If you get a 45 on your MCAT, you don't need to get a special pin (that costs $80 on top of the $200 it cost to take the MCAT) to get into the "45 MCAT Honor Society." All you need is that paper they send you in the mail.
That aside, PBK is still very prestigious, so if you can spare the 14 characters in your AMCAS awards section, and the $80 (which, if you're applying to med school, you certainly can), why not? But don't beat yourself over the head if you didn't do it. After all, you still have that super-high GPA resting firmly on your application. Nobody can take that from you (except for my quantum mechanics professor... he took about 0.20 of that from me).