You can get them ahead of time - in fact I think you have to if you want them. There is no limit to the number of coupons you buy. The coupons have a maximum value so it isn't as though you can get a $10 drink for a coupon alone. I believe they are good for drinks up to $5.25 including tip. Anything over that you have to pay with your cruise card unless the bartender is feeling charitable. Anything less than the maximum value and you don't get a refund of the difference, so you lose money. In other words if you order a beer that costs $3.25 you just lost $2.
It is a hassle to carry them around. You also can't turn unused ones in for a refund. Once you buy them you are stuck with them. You are on a cruise vacation, so why rummage around for a coupon when you want a drink?
The coupons are supposed to be a gift option and not a primary method of buying drinks by the people on a cruise.