I was going to say the same as BG - it's not rude at all as long as it's on the invite or you let your guests know beforehand so they are prepared. I have been to heaps of weddings where we had to buy our own drinks and it doesn't bother me unless they don't tell us but I always take cash with me to a wedding now just in case we have to buy our own drinks. At our wedding, my parents paid for softdrinks and juice for all our guests and we had punch on the tables. Alcohol was available and people could buy their own. We provided drinks for toasts.

I went to a wedding a few years ago where we were given drink tokens - the B & G paid for three drinks each for everyone and after that we had to pay for our own. They put them under our placecards on the tables. It worked okay too as another option.