You also need to compare the prices with a cruise-only travel agent (human, not OTA, preferrably a top producer.)

We cruise a lot (4 this year included 2 TATL and 1 European). Have done over 30 cruises across lines but primarily with Princess and Holland America. I have found that if I book thru my agent, the prices are always cheaper than any airline's offering. It essentially boils down to, do you want Cash in your pocket or miles in your account. It really boils down to, you "pay" for those miles by paying a higher price (can be a few hundreds higher).

After comparing the quotes a few times, these days I simply delete the junk mails from AA and UA pushing their cruise bookings which actually are done by one of the cruise-only OTAs.