Given the very little detail we actually know about any 4.3 content, it would be unwise to assume that the improvement to Vendetta will be as 'useless' as it is in current content.
The st bonus could still be useful, say, for Lord Rhyolith type encounters, where the extra seconds could mean getting some extra boosted attacks in despite the need to target switch constantly.
Never said it was useless, but with Vendetta being such a long lasting debuff it's not always as useful as dance or AR. Yeah you still get some of the bonus on a fight like Ryolith, but the bonuses to AR and ShD are better because they stay with you even when you switch targets.
I'm not complaining, just pointing out that mutilate is gonna get screwed unless there's a lot of fights where we can stick on 1 target for 39 seconds at a time.