Many people talk about TBN as if it's something you can always break on demand. That's probably true when you use your (10%) ally bubble, as it's something like a 5k barrier.
Your 20% barrier will only break on specific points in a fight, like cleaves. This is one of the reasons why a duration extension is a bad idea. It encourages TBN's use on autos, rather than specifically on big hits.
If the barrier was 20% regardless of target and the duration was fixed at 5s, you could easily make it a dps gain as a reward for knowing the cleave timings of the fight. Right now, you could shield every big hit on your co-tank with it (and it's extremely fun to), but you probably don't want to for potency reasons.