Hi John,
I have a ZD 331 mower with a rear discharge deck. We operate on the west coast of British Columbia Canada, and it's pretty wet out here most of the time.
The rear discharge deck works well in about a weeks worth of growth but, much more than that and the clippings tend to get left in little wind rows. As for the grass packing under the machine, no that has not been a problem in my experience. If you are doing lots of cutting in wet grass, a thin layer will stick to the under carriage, but that's easily hosed off at the end of the day. This style of deck is a great tool in the mowing business, because you can mow in both directions without spraying clippings on anything and, it minimizes the chance of throwing a projectile into something really expensive. The deck is also built super heavy, weighing in at about 400 pounds. On another note, I happen to have a 60" rear discharge deck for sale. The unit is a 2011 model with only 200 hours on it. This deck came standard on a 331 I bought, but most of our customers wanted the clippings picked up, so I bought a side discharge deck to accommodate a grass catcher. The new price for these is about $2400.00, but I am selling mine for $1000.00 firm. Anyway, I hope this helps you out.
thanks,
Westcoastkris