Let me do some math for you... 10 beers x $8 × 4 days X 2 people = $640. I realize that I'm not using "new math" so the answer could be different these days.
That said, I have slowly come around to thinking if you feel something is going to increase the enjoyment of your cruise, and you can afford it, then spend your money as you see fit. When I think about what I spend for internet at home vs what I pay on the ship it drives me crazy. However, having internet adds to my cruise enjoyment and I don't have to take out a loan to purchase it.