They make a fair bit of noise, so they can actually distract from the rest of the stall. People can just watch the TV for a while before moving on, rather than having to ask at the stall. They are a major hassle to hire, set up, get electricity for, and take down again. There are always problems with sun reflecting off the screen and rain. It also does not seem to draw in new members as expected.
Most new members seem to be attracted by other things.
All in all, I feel they are not worth the investment and trouble. If you can get one free on the other hand, they are probably worthwhile just for the attraction. Make sure you have a long extension cord, about 30m, and access to power. It is much better if there are discs for sale and to show people. It is easier to keep people in the club if they have something to practise with.