Yeah NB can definitely tank. However as a healer I'd take a DK tank anyday over a NB (unless the NB player is playing sinificantly better than the DK player)

The NB tanking mechanic relying on keeping the boss standing around like a fool thanks to cloak and outhealing the damage taken with the siphon thing is rather nice but at the end of the day this means that I've little to do when things go alright for the tank, and the situation becomes out of control when he cannot attack or cloak for whatever reason.

Basically that makes the combat rather unpredictable and spiky.

The same logic applies to the NB healers I guess.

Again I'm not saying people cannot build their toons the way they like while staying successful, but in many cases the combat will be smoother if people stick to their class main afinity.

However I do think that creative builds might be very interesting for trials since
The most optimized way (read: with high likelyhood of success) =/= fastest way