In X-Com (the 1994 original, not the new game), you start with some heavy cannons and and an auto-cannon. Are either of these weapons still tactically advantageous in any situation once you've got enough laser rifles to equip your whole squad?
The main benefit of those cannons is the fact that you can load them with HE and Incendiary rounds. I tend to carry them on only a few of my troopers (that usually have high strength to be able to carry so much gear). On night time missions, the Incendiary rounds are a great way of creating lots of light (but then so are the cheap light sources that you can throw). The HE can be used to blow up walls and such.
Once you get blaster launchers, however, they become totally out classed and not as useful.