I've taken the MCATs before and I'll have to say that the material on the mcats is a LOT more in depth(in diff topics) than those on the PCATs. Thus, the mcat books contain more information than is necessary for the pcats. The MCAt for bio emphasizes quite a bit on physiology, anatomy, biochem/cell bio where as the PCAT bio included a lot of biology I did in high school (plant biology/botany, zoology, etc).
The chemistry for MCAT also emphasizes a higher level organic chemistry, with grignard, SN1/SN2 E1/E2 etc versus the PCAT chem which was mainly basic general chemistry and barely any ochem.
PCAT verbal, definitely a lot easier to break down than the MCAT verbals. The passages might have been shorter (thus more time to read and analyze) versus the mcat where you really feel pressed for time. Using the MCAT verbal section to study could possibly decrease your reading/answering time on the pcats, enabling you to go over your answers at the end.
And as the other poster said, if you're taking the test soon, then it might be best to stick with the PCAT book cuz using the MCAT prep books to study might be overkill (with random knowledge you don't really need)
Another thing: I found the PCAT books not very helpful because they turned out pretty different from the exam. They were also a lot easier than the exam. I used Barrons, and...some other pretty popular one. On the other hand, the mcat books explain things pretty clearly and they have a lot of background info just in case you're a bit rusty on topics.
Overall, I think using the MCAT to supplement your pcat studying is fine, but don't get caught up if you miss stuff because it's probably not on the pcat anyways. They are two different exams that test different things.
I scored in the 97/98% for the PCAT science/maths, and I think the majority of the information I had was leftover from what I remember of mcat studying 3 years ago...