I'm in full agreement with your comments about passengers who either don't eat dinner in the dining room or skip out before dessert on the last night of a cruise to avoid tipping the waitstaff. Anybody who can't afford a fair tip can't afford the cruise, either.
On the other hand, I'm quite miffed that any cruise line would be so brazen and so tacky as to leave tip envelopes in the passenger staterooms. In my experience cruising with Princess, there has always been ample notice that tip envelopes were available at the Pursor's Desk.
I can "pro and con" the idea of the cruise line knowing the amount of tips and have not used the vouchers in the past, but there is a significant advantage for purposes of income taxes. With tips paid through the voucher system, the cruise line can report the amount of tips given to each member of the crew accurately. This avoids the problem of taxing authorities like our Internal Revenue Service trying to claim that service personnel receive more in tips than they actually do receive and thereby assessing taxes that are not legitimately owed. Of course this is much more of an indightment of our system of tax collection than of the cruise lines that have instituted the tip voucher system.