It is a smart idea to be seen at the same events which your prospective clients are attending. I honestly can't tell you to what extent it would pay to have a booth as a freelance translator (if you know, I will invite you to write a guess post about your experience), but I know the feeling — and the fruits — of being the only translator in a room full of lawyers with international practice. A significant part of my income for 2013 came from that source.
This is not something I came up with. Experienced lawyers have suggested the same to less experienced lawyers for quite some time now, and so have successful translators. To win a client, you need to think like a client, go where a client goes, talk the talk, walk the walk. If you walk the walk, your talk will also be more convincing.
At worst, you'll know what the atmosphere at an expo feels like and will feel more at ease when you need to interpret during one or translate some materials for it. You'll pick up some other information too, make some contacts, leave some business cards (you never know where those will wind up). I don't really think it can be a waste of your time — much less a total waste — unless your language or specialisation is so rare or disconnected from business that you have nothing to gain by going. Or unless you've already gone dozens of times and concluded, empirically, that it doesn't work for you.
Follow-up (for compulsive readers, inquisitive bees and bored minds):
A lot of modern services marketing is based on understanding your clients. You need to show them that you understand them because without understanding them: 1) you can't really address and solve their problems (and the emphasis tends to fall on that lately, as opposed to delivering traditional services); 2) you can't really relate anyway, or gain trust. Trust is really important in turning 'leads' (clues) into 'conversions' (new clients, jobs and so on for you), as marketing people call them.
Actually, there is even a chance that someone at the expo or event will be talking about the problems, challenges and opportunities the attendees are facing. Chances are you'll see consultants and coaches at work there turning the attending businessmen and businesswomen into their clients. You could pick up some ideas and use them, it will cost you nothing (no copyright infringement, of course). Translators aren't typically consultants, but our role and our relationship with those clients who care is somewhat close.
Some other ideas that are coming to my mind right now are that you can pick up some authentic blurbs (leaflets, brochers and so on) for reference when you translate similar materials for your own business clients or when you create your materials in which you are trying to relate to clients from the same industry. Speaking their exact language will make it easier.
Obviously, you have more to gain from this when you're new than when you're thoroughly established and going to exposes and events in relevant industries will bring you no new opportunities (whether as contacts or knowledge).