As Madara put it, Obito is about to begin the *Eternal Tsukuyomi* . Well, the 10-tails has a pretty large eye, and considering the blast radius of a tailed beast ball, I don't think he is too concerned with making himself a larger 'target' per say.
The 10-tailed beast itself flicked a tailed beast ball away as if it were nothing, so I see some reason in going into tailed beast mode for Obito, as it would probably aide in his durability if he is attacked before the technique.
However, I only have on question: if Obito cannot use Tsukuyomi, since his mangekyou technique is Kamui, then how is he going to perform the Eternal version of Tsukuyomi ?