A few years back my boss gave an open invitation to everybody in the group I was working with to go to Boston for a conference. He'd pay for the hotel and conference fee, but we'd pay everything else. It sounded like a good opportunity, but the costs I'd have to pay struck me as a bit much, considering I was a student at the time. There'd be the plane tickets, food expenses, spending money for touristy stuff, car rental and hotel parking fees. The last two struck me as putting a exceptional dent in the wallet. Then a few friends going on the trip assured me I wouldn't need to rent a car. I didn't quite believe them so I checked with some friends living in Boston.
"Yeah, you don't need a car. The subway system is fine here."
I wasn't aware that Boston even had a subway until that point, (if it got mentioned on Boston Legal, I missed it) but I took her word. I arrived in Boston, hopped on the Silver Line and was downtown in minutes. In all fairness, the Silver Line was a bus, not a rail car, but it was a bus that went directly to the transit center on a dedicated roadway (in other words, Silver Line buses were the only vehicles allowed to use that road). From the transit center, it was easy to hop on the subway and go anywhere of interest. I had no problem getting around all weekend, and managed to check out everything from Fenway to Harvard.
I'm pointing this out because the idea of doing this in Cincinnati right now is laughable. If there's a way to get from the airport to downtown Cincinnati without using a car, I'm willing to bet that most people don't know about it, or tell you off the top of their head where it would drop you off.
Whether or not there is a cheap and easy way to get from the airport to downtown Cincinnati, the second problem is getting around. If you don't have a car, your options are to take the cab or bus. Cabs can be a bit pricey and in Cincinnati, most of the time you're better off calling ahead for one than trying to catch one at a taxi stand. I suspect local taxi numbers are not something you're average out of towner is going to consider picking bringing along in advance.
As for buses, Metro stops may be plentiful, but they offer little useful information about where the bus actually goes. Your average Metro stop only has a sign indicating listing the route numbers, not destinations. For and out of towner, those signs might as well list the numbers from Lost.
So for people visiting from out of town, it would be exceptionally handy if we had a mass transit system in place that would be easy to use for somebody who doesn't know the area. Ideally, this would be the streetcar system already being proposed.
The main advantage of such a simplified system like the proposed streetcar is that it's easier to maneuver. When giving directions, you only need to tell them to find any station and get off at the right stop. Compare this to a bus system where the first step is finding a bus stop that even a bus going on the route you want it to go on.
And Cincinnati does get a fair amount out of town visitors. With all the corporations based downtown (P&G, First Group, Kroger, Chiquita and Macy's to name a few), they definitely bring in a lot of out of towners. Then there's the conventions, the sports events, concerts and so on. A lot of these groups (especially the business people, I'm sure) would easily take advantage of a system where you never need to get in a car or go to an outside source like the metro website to figure out how to get around.