So, what would Tobi gain out of it? A few seconds he couldn't use for anything? It might matter a lot if he was facing Naruto alone, but since he's not, there's not much use for the trick. Naruto+Kyuubi would just power up again while Tobi was busy with the others. For all we know the much better synchronization between Naruto and Kyuubi might make suppressing them with a sharingan much harder as well.