If you have both Valetini and Isi you nearly double the workload Hooper has to do at ruck from the back row, and you probably change the dynamic of the tight five as well.
Swinton produced some really good plays last night and his physicality was top notch BUT he lost the ball twice on exit (one of them looked dodgy as, mind) and gave away a penalty that could have been a yellow card.
Importantly, Swinton's ability to switch between support player and ball runner / link man allows Hooper some breathing space. If you have Wilson at 8, he also has to perform some of these roles so you lose a bit of his ball carrying.
There are a few ways to balance our back row. None of them are right or wrong until the final whistle has sounded.
What I like about Swinton is he doesn't give a shit who the opposition are, how hard they run, how big they might look. They're all just targets to him, and when he gets it right we don't have a better mongrel backrower in the country.