Agents I worked with are slimy vermin that crawl in the mud at the bottom of a dirty river. Some were okay, but a few tried to make acts sign that they will book only through them. NEVER sign that deal. Demand that they book you on a job by job basis, where you have right of first refusal. That keeps you out of Bob's Country Bunker.

As far as concerns about recorded backing tracks, agents will book you to go play spoons with your dirty feet on the sidewalk in front of a homeless shelter as long as they get 15% of what you get paid. The downside is that they will try to coerce you into taking such an awful gig. (see: "vermin" above)

Just make sure you are not bound solely to them, demand a contract for every job they get you with numbers clearly stated. No percentages. Numbers. "Artist will be paid $350. Agent commission will be $52.50." That protects you if the client offers to tip you $100. That is over and above your fee and the vermin, er, agent is not entitled to 15 of those dollars.