Replacing Price Pfister cartridge bath faucet

Hi all, my upstairs bath faucet is leaking. I know it's Price Pfister, and I have (close to) the diagram for it. What I'm wondering is, when I turned off the water main, I opened the valve to let all of the water out of the spout. But when I pulled off the plate and unscrewed the handle assembly, there was still a lot of water sitting in there. I had to push a towel in the opening to catch it.

Is that normal? Is it bad to let too much water fall in behind the enclosure (I'm assuming yes)? But is a little OK? Thanks.