Let's face it, BRD's don't even give up their shooty time to use their decent songs like foe, when are they ever going to use the warden's paean that barely works on anything?
The only way I can see this move being useful is if it worked on debuffs that actually mattered and by stopping it would allow the raid to significantly increase dps.
I'm talking about tank swap debuffs. If BRD could subvert tank swap mechanics then we could bring another dps instead of a second tank.
E.G phoenix's revelation, if warden's paean prevented the revelation debuff then Phoenix could be solo tanked, allowing the group to bring another dps, or even just allowing the OT to full time dps which would significantly increase raid damage.
Or Titan ex, if paean prevented vulnerability down then the same thing could be done there.
(I'm aware both these examples are 50 content and paean isn't available for them and that Titan can be solo tanked anyway, these are examples of the types of things in future content that bard should be able to prevent with paean).