You loose the capability of concentrating firepower. x)
And then, if you spread too much, your adversary can play the "denied flank" and screw you. x)
And then, again, DoW maps don't allow you to spread too much, meaning that a well placed Solar Pulse can force most of your squads to get back.
And, to make it clear (before you start saying that it can be easily countered) for the 3rd time at least, i'm NOT saying that one ability/unit (Solar Pulse in this case) can take out everything on its own.