Hi Meiji!

1. Yes this affix affects transfer time in both ways, contrary to Reduced transfer time from WP to Rage that works as said (you can find this last one on every staves, every catalysts as a malus and on unique shoulder).
   2-3. Resource in transfer cannot be use, so yes it is useful for everyone to have it if you have trouble with transfer times. If you spam too quickly your resource and/or use skills with bad transfer time you may end with all your resource locked in transition forcing you to stop using your skills (and so be completely dependent on the transfer time to cast your skills). Catalysts gives also a malus to transfer time so it's more likely to happen with a hybrid build, but it's just a counterpart to the huge advantage of not having to worry about resource generation anymore.

Otherwise if you don't have these kind of problems it's completely useless as it means that you always have enough resource available to do your stuff. but it can be usefull for mages to have a lot of unspent rage instead of locked willpower and so benefit from the Sarisel affix +X% damage per rage unspent, but that's pretty much all imo