Are heavy cannon and auto-cannon still tactically advantageous after acquiring the laser rifle?
Solution 1:
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.