I feel that the balance has to be kept among the Human weapons. If you notice, the main rifles are the same way. You have 4-types, each with their own strengths and weaknesses. The Ar, Br, DMR, & SAW.
The Ar is your close to mid-range equalizer with decent sized clips, fast firing speed, and decent damage. The BR has good damage, also a decent fire speed, but can get headshots while in mid range. The DMR does long range fights, fires slowly, but has better accuracy and damage than the others. Finally the SAW, which is powerful, with a lot of ammo with the trade off of fast firing and short range.
You'll see the same balance between the RL, Sticky, Spartan Laser, and Railgun. The laser and the rockets are good against vehicles, while the sticky only works on light enemies and some light vehicles. The railgun is your heavy-enemy friendly counterpart. It fires faster than the spartan laser, and hits sooner than a rocket or a sticky. It just isn't as powerful as the rocket launcher or the laser. On the distance scale, it trumps the rl since rockets are so slow, ditto for the sticky, but the laser is for super distance.
Now, to answer your main point, why use the railgun when rockets are so much better? Situational. If you have both, use rockets on hunters, wraiths, and flyers. The railgun should only be used on ghosts, covenant, prometheans, and spartans. If you have both, odds are you'll be facing vehicles. If not, of course use the rocket launcher. JUST DONT BE SCARED OF HAVING OPTIONS!
It's Spartan Laser: Low Calorie edition. Sucks against vehicles but is handy for anti-personnel compared to the laser since unlike the laser, the rail gun is at full power a few seconds before it forces you to fire, meaning you have a larger window to line up a shot and you also have a higher potential rate of fire. The down side is the projectile is not quite so instantaneous as the laser, and that it's worthless versus armor.
Personally I like it, especially on the Spartan Ops missions where it has infinite ammo.
Is there something about the way I type that makes it sound angry? Maybe it's just something about the way you read.