It slows you down so people don't use it
The alt-fire attack gives you a forward dash before slowing you down and the attack itself has about 40% more range. It's effectively much better at picking individual targets although you don't really want to spam it :)
When i use it myself, i usually use the alt-fire attack to get in range of my target then i use the regular attack if i ever miss or if there are more targets to clean-up afterwards.