I've never been a big fan of Slayer - walked out when they toured with Megadeth (I told myself I'd leave after War Ensemble and luckily that was the second song) - and I've never heard Exodus, so no idea what Holt is like. I'll probably see them at Soundwave though, just so I can say I saw them with him. Someone's bound to be impressed one day.
I don't understand the fanatical love of Slayer, but yes I am quite a fan, and I think they just got Gary in because they need a good thrash player, and someone who understands it is a very temporary prospect. Plus I am sure being mates with the band will help too