If he's half-way decent, he'll take a step to the side, watch your rocket fly harmlessly by, and then continue on his merry way. He'll stop to pick up the weapon you dropped when he bagged you first, though. You didn't aim at his feet. If you would have hit the floor or wall next to him, he would have been guaranteed to take at least some token amount of damage. That damage might have been enough to do him in.
Splash damage is better than no damage at all.
Next point - Cleaning up after yourself. Sooner or later, you're going to notice that you've been getting pretty handy with the railgun. The railgun is a beauty piece of work, because it delivers 100 points of damage instantaneously to anything it hits. It also keeps going, so its possible to bag more than one person with a single shot. Often times, unfortunately, a single railslug isn't enough to take an opponent down, since a single armor shard or 2-point health pack is enough to keep you alive and kicking, should you be on the receiving end of a rail.