I'm currently putting heavy bolters in the squads and autocannons in the command squads.
It's what I do. (see http://www.40konline.com/index.php/topic,93285.0.html
Yes, you could
put the lascannons and missile launchers in the infantry squads, but a heavy support squad costs the same price. The difference? with a 10-6 guardsmen ratio, it's going to take the enemy 5 kills to wipe you down to one lascannon while it's going to take 9 to take a full squad down to one (assuming that neither squad retreats). So, while your squad is going to have twice the survivability of a heavy support squad, it has only a third of the firepower (1 vs 3 lascannons). Plus, you're absolutely right: a squad with a lascannon is firstly never going to move (which is only a problem with certain army types) and secondly, it's going to waste all of those lasgun shots. For me, it begs the question of why bring the guardsmen if you're not going to use them (apart from meat shields that is. Remember, a good general does not lead his army to destruction just because he knows it will follow).
As well, if you put your lascannons or missile launchers in your squads, then you have to put your heavy bolters in your heavy support squadrons, which I don't like (see http://www.40konline.com/index.php/topic,93341.0.html
In short: go for it.