I personally cancel that shield when it is casted directly. In some situations when you can't charge and start only with bloodrage, a few lucky dodges in a row and find yourself with a depleted rage bar even in malygos. Add this situation to some rushy dps and things can go ugly. Other than that it's nice to have a shield sometimes.
I think alot if it is situational based. For (heroic) fights like Sapphiron where the raid need to possition themselves before hitting their nuke buttons, I can't see it being an issue. For something more intense where the enrage timer is your worst enemy (like Malygos), I would agree that a shield isn't as handy as you'd like it to be.