Combat Squads can also be useful in large games to allow you to split into two units of five bodies with two Missiles each; these can split fire and are harder to bring down than the usual five bodies, four guns setup. However, it's only really worthwhile if you're using up all your HS slots and still need some extra flexibility/durability.
I don't generally hold with splitting weapon types, even the Las/Missile split I see fairly often. Yes, you can give the Signum bonus to the Lascannon, but it can force some weird target priority issues (like when shooting at a 2+ save model) and I don't feel the additional points for the Lascannon are worth the small boost in performance it gets against most targets. We have abundant Meltaguns, LasPlas Razorbacks, and AutoLas Predators all as good ways to destroy heavier vehicles and infantry units; I don't think sinking the extra points for Devastators with Lascannons is a particularly effective plan.
The concern being, however, that terrain can block LOS or give cover against the Dev's target, whereas our other platform performing similar fire-support roles could maneuver around it. As a static unit, they are also unable to stave for time against enemy melee and are unusually vulnerable to backfield assaults (from Ymgarl, Wolf Scouts, Kommandos, DSing units, etc.)
Simply discounting them as "not mobile" is silly though. Mobility isn't as much as an issue if your threat range extends to every corner of the board.
It doesn't make them bad, it's just a balancing factor- Devastators are your best firepower-per-point buy in the codex, but they have weaknesses of their own that must be considered, just like all units do.