I researched this when I set my trailer up. After considering all the additional plumbing, space, carbonation tanks, syrup, cups, straws and ice along with experienced people telling me I'd be switching out tanks and syrup right in the middle of busy time I decided against it.
Looking back I am glad that I did. If I vend a festival we sell a lot of drinks say 15-25 cases in a weekend. On the other hand if I set up road side on a Saturday we will only sell about a case and a half. I only do s few festivals a year 4 or 5 so the fountain would not pay off.