The Princess Plus package is much more attractive, although at first it looks expensive. £50pppd, but when you take the autotips into account, plus free wifi, the actual drinks element comes in at well under £40. That buys you 15 alcoholic drinks a day, up to $15 each, plus soft drinks, coffee etc.
The big advantage over P&O is that far more drinks, including cocktaiks, fall within the $15 barrier. And if you do want, say, an $18 glass of wine, you only pay the difference, plus 18%, so a total of $3.54.
P&O must make money out of the drinks package, and I suspect they would get a greater uptake if the £6.95 limit was increased, and the Princess policy regarding more expensive drinks was adopted.