You know after a random sampling of European based cruiselines, I can tell you that the 10 a day is a US thing.
P&O suggests 3.75 per day,
Ocean Village, states that it is included,
Thompson says 4
Costa states 6
Easycruise, none, but there is no one to tip as well since nothing is included.
Considering the fact that the crew doesn't make only the $50 a month, they are guaranteed a minimum according to the lawsuit a few posts back! That means it works like a restaurant where they may pay you less, but if you do not make it in tips, then the owner does. Since they seem to make up the difference, then I don't see a reason to tip in addition, sock it back to the cruiselines.
If they want to increase the fare by $70 dollars, then they should go for it, and if you complain that it will start the whole tipping cycle over again well, then you only have yourselves to blame for doing it.