I think it's a good idea. Treat it like an interview. When I was at 9E, I went to the Spirit job fair in Oct 2011. No matter what job fair you go to, if any big players are showing up (Spirit, VA, United, US Airways), there will a huge attendance. Waiting in line was the hard part. You overhear some other pilots' qualifications and might even think 'I don't stand a chance.' But it's a great opportunity to meet/greet the pilot HR team and the pilots on the recruitment team. Keep your contact professional, and don't babble on and on as if they're your best friends (and likely they'll make you feel so comfortable that you feel like they are your best buddies). You could dig a hole if you're not careful. Goes back to that advice, treat it like an interview. At the Spirit job fair, all the pilots I saw had a suit on. You can't go wrong with that, but at the very least, have a shirt and tie. Make sure you have several copies of your resume printed on RESUME paper, and bring any LORs.
My result was a little disappointing, I met the right HR person and the ACP who was involved with hiring, I had two good LORs, and at the end I heard nothing. Which is sad, because later I found out that almost everybody who went to the Spirit job fair who had the minimums or higher did get a call to interview. I didn't, but one closed door eventually will lead to an open door, and I landed my current job without attending any job fair and no internal recs.
Good luck!