I will agree that getting into races is much easier with an agent (have had one) but getting into them without was is not impossible. I currently do the job myself and it isn't so bad. You do have to be a little diligent in making sure race directors return your emails/calls but some are better than others. I have found that if you get into a race once it is much easier the next time as you already have a connection and hopefully you have run well there before. Track races seem to be a bit harder. Getting into a big meet like Stanford, Payton Jordan, Pre can be pretty difficult. Ideally your coach might go to bat for you here. In my personal opinion, unless you are trying to get into some European meets, an agent isn't necessary.
Plus, I guess this is just how I feel personally, if a race director won't pay your travel you can always go to the race, clean-up some prize money to pay for the trip and then be on solid ground the following years. If you have a bad race and don't make money, well at least you didn't piss off the race director for bringing in a guy who ran poorly.