Yeah, but for example free death coil for UH DK does not consume runic power but still increase damage done by Valkyr battlemaiden (so it counts as "consumed" runic power).
I assume (like with other legendaries) that it's intended to make other talents more interesting. Some legendaries are worse with some talents, and some (like this one, maybe) doesn't work at all with specific talents.
Especially this one since, like you say, if it works you'd be able to keep it up constantly.