Why does Obito only grab some people when he teleports them to the kamui dimension?
Tobi couldn't attack and defend at the same time with Kamui. 
His strategy was to allow his opponent to attack, but by using Kamui, the attack would fly straight through. 


If Minato didn't have the teleportation, Tobi would have won.
Tobi's strategy is actually quite interesting, as he didn't use the kamui in the way that you thought that he would. I think that this is Obito's way of doing. He could use the Kamui in that way but chooses not to, because his strategy is more tactical.