With the sharingan, each eye does different things. Like how one eye of sasuke can cast ameterasu, the other eye however can basically absorb it back which is shown in the fight against B.
So, one of Obito's eyes can make himself intangible, the other can send other things away to another dimension. If the boulder landed on the other side of his body and kakashi got the other eye instead, kakashi would have the abilities obito is shown. That and naruto, sasuke and sakura would have never have even touched a bell, would have been hilarious really.