I am by no means a master MM or anything, but I thought I would offer my two cents over my experience in the early leveling process of beast/nature. I chose the powers for thematic reasons, and did great until about level 10 (soloing missions in Praetoria). After that, my pets and I started getting squished, a lot, and I found the character a bit unenjoyable to play. So I rolled a beast/time MM and boy did I find myself enjoying her! I know /time needs levels before it really gets going, but just from this early perspective I already thought the play experience was much better. Granted, I rolled blue instead of gold and joined DFB's and Posi TF's because I wanted those experiences and to try and see if I could maintain my pets in a group situation. But the times I was/do solo, I feel more comfortable having /time than /nature. As always, YMMV, and other, far more experienced MM players may have gotten much better use out of the beast/nature combo than I did.
Edit: And after coming to the boards here, I think I'm going to re-roll out of beast/ entirely. I enjoy them thematically, but have been a bit underwhelmed with them in game, and after coming here to the boards, I know why. I think I will be re-rolling my beast/time MM to thugs/time since I already have demons/thermal.