Here's how I use Terpsi in practice right now with WHM Haste only:
* 300 TP WS -> melee for 2.5 minutes
* At 2.5 minutes, I stop WSing and rebuild 300 TP. Typically I No Foot Rise -> Reverse Flourish as well to get there faster.
* I get 300 TP right around 3 minutes, at which point I wait for AM3 to drop and reapply it.

Letting us overwrite would really just save me a few seconds of sitting there waiting for AM3 to fall before I can reapply when I get 300TP before it wears off. It won't really improve my damage potential very much. It'd just improve how smoothly I can play with a mythic. It's annoying to have to wait 5 seconds so your activation WS won't be wasted or, worse, the times when you don't think about it and end up wasting it.

It's not a very large buff and applies to a subset of weapons that aren't really a balance threat because they're so rare. I don't see the logic behind SE's response.