This is all true. This is also my point. For no spec does Disturbance seem to be anything more than a move used grudgingly, and that's just for TK/Lightning. The other two specs just ignore the power altogether. Now if it was awesome for TK spec, that would be different. There are plenty of powers that are pretty much made for one spec or the other. Mind Crush seems like a primarily Balance spec power for example. But Disturbance really isn't much TK other than to proc effects.
It's a filler move that procs other (better) abilities for TK, nothing more, nothing less. You can use it outside of TK spec, but its usually better to use something else. Now that's not isolated to Sages, that applies to a lot of classes. TkT is only situationally useful for non-tank Shadows and Rupture is primarily for Annihilation Maras while Vanguards will rarely use Full Auto.