I get a few of these offers even though I'm not into hair - I don't do them. Like anything you have to look at it carefully. Remember, their business is to sell you space at a wedding fair and that's all. I'm sure someone will be able to post their own experiences of going to a few.
The bottom line is are you interested in doing wedding hair for people in the catchment area for the wedding fair? Is that what your business is about.Do people go to different hairstylists for their big day or do they use their usual stylist?
You can also look if the event has been publicised well and is it hosted at a good venue that portrays the right image?