I wonder, would it actually even cause any balance problems if TBN were to be extended all the way to 10 seconds? Other equivalent tank skills have short durations because, being percentage based reductions, the amount mitigated by them increases the longer they last, as a result of soaking more hits. But TBN is always the same total mitigation, a fixed amount of 25% of your maximum HP, no matter how long it lasts, so increasing its duration just makes it more flexible for fluff damage, and less likely to punish you.
