If you want to be a NIN, I say go for it. Some adjustments are coming in 3.1. Mudra adjustments are likely to result in at least some DPS increase, and TP adjustments will make you less of a burden on the raid's DPS. (I also wonder if we'll see further adjustments, such as to AoE, but they haven't specifically said anything about other changes). As others have noted, though, NIN's strength is best realized through Trick Attack and their adaptability in fights with long phase shifts and the like.