I think the primary reason for sealing primers would be to keep moisture out of the priming compound and powder. That said, unless you're reloading shotshells for duck hunting around open water or for ammunition to be stored in less than ideal conditions, I can't see how much of a benefit it would be.
Formerly TN Lone Wolf
"We are only as strong as we are united, as weak as we are divided." - J.K. Rowling