Some of the new PPC are pretty cool... the Kel-Tec Sub 2000 folds in half, that is a neat trick for stowage. The magizines are the same as your pistol(if you work it out that way), they are smaller and lighter than AR or AK mags, so they take up less room. That's a plus! I think that if ones needs/situation requires something smaller than an AR/AK type platform, it's great! Everybody will have slightly different needs...
Of course, PPC aren't going to be 300yd shooters... but, they do provide a 100yd weapon with reasonable accuracy. Much better than lobbing 9mm from a 5" Glock... You won't get much in the way of penetration, at closer distances you'll see some benefits there... within 25yds, vehicle bodies will be penetrated easily. If you look at out of the ordinary PPCs, like a PPS-43... chambered in 7.62x25mm, you have increased penetration over 9mm/45acp at all comparable distances. I know, I know... a PPS-43 is a "sub-machincegun"... not in semi-auto only though.
I think that all weapons have their respective places... however, not everyone will have those "places" for weapons.