I don't know about Tampa's streetcar ridership numbers, but it has been a major catalyst that has spurred over $1 billion dollars in the redevelopment of the Channel District. That alone makes it worthwhile, imo and justifies the cost to build it, if compared with any other form of transit (ex. roads, bus, etc.). However, as you say, it is clearly a tourist attraction instead of a true mass transit system that residents rely on. It's speed is one issue, but another is it does not go where residents live and work, like the lines in New Orleans and Memphis do.
For example, it does not serve the heart of the Central Business District, Hyde Park or even the University of Tampa. So overall, it suffers from the same thing that our skyway does....inferior route planning.
Btw, I thought they were getting ready to expand the street car route up Franklin Street? Also, they are talking about shutting down their trolley bus before the streetcar.