Of all the cruise lines we've been on, the one with the best service was the one with a strict "no tipping, tips are included" policy. The second best used the envelope system. I have to conclude that there is much more to the issue of "good service" than the manner of compensation of the employees. After all, with so many lines going with some form of "open seating dinner", you really have no one to tip with the envelope but the room steward.
I will be clear on the fact that I have no Carnival experience, and therefore cannot comment on this line's general standards of service. I do think I can comment, however, that our experience has been that the matter of whether tips are given in envelopes, given via charges to our accounts, or included in the fare is not relevant to the level of service we receive.