Ok, since I am bored and the replies are getting sillier and sillier, here is the economic argument:
The best economic argument can be made using consumer surplus, and has nothing to do with the fountains as an ecomonic stimulus for either the Bellagio or Vegas in general.
Basically, there is enormous
consumer surplus created by the enjoyment of thousands of people seeing the fountain show every day. This consumer surplus far outweighs the negligible
marginal cost of running the fountain. Paving over the fountain would cause the entire consumer surplus to be lost.
This is purely a cost-benefit analysis of the fountain istelf, and has nothing to do with whether it is good for the Bellagio or the city of Las Vegas. The Bellagio pays for the fountain because they believe that they can capture some of that consumer surplus in the form of increased gaming, restaurant, or hotel revenue, but even if they are not doing this it makes economic sense (in general, not for the Bellagio) to keep the fountains operating.