It's a big deal at several schools (ucsf and ucsd to name a couple) They want to know that it's what you want and you can only know that it's what you want if you've been exposed to it. I'm of one accord with this idea. I kinda think it's crazy to go into it if you don't know for nothing else than: What if you find out you hate it?
Granted, i've met a bunch of people who had little exposure at all to pharmacy. The student that iv'd me at one of my schools was clearly very smart, but she knew next to nothing about how pharmacies work. So, it's definitely not a must, but a very wise choice.