Wow...
You do know that if you used strategy, like attacking from multiple sides, flanking your enemy, and stuff like that is not possible if you have them all selected?
You always keep track on your units, no? I mean, you don't forget a single one? That is important.
I meet a newb so nooby that I almost laughed. He saw that I was teching air units but he kept on training ground units, he never expanded too. He destroyed my first base and did not see I had two other bases on the right side of the map. XD
I had scouts everywhere, I massed up a new army, I harrassed him and then all I needed to do was to use the attack move button over his base. XD