It is inflexible because you 1. break your combo with it and 2. even if Clemency didn't break your combo, you would still delay your rotation by 1GCD every time you used it, making you slowly but surely drift even more out of party buff windows than you currently do.
So we can not work with the assumption that the "new" Clemency is used outside of Requiescat, because Req windows are the only time where you can use it without screwing yourself.