Personally I want the seniors on my team actually delivering on the really hard stuff.
Helping juniors do their job is great and all, but you still need experienced people to work on the hard and complex stuff that juniors can't because they don't have the knowledge/experience/people-skills. No amount of pair programming can replace that.
You don't want to be in a situation where you have really really well implemented low-value features, but the high-impact and high-priority hard stuff was not done because some of your most experienced folks were helping the less-experienxed people write effective unit tests or whatever.
Helping juniors do their job is great and all, but you still need experienced people to work on the hard and complex stuff that juniors can't because they don't have the knowledge/experience/people-skills. No amount of pair programming can replace that.
You don't want to be in a situation where you have really really well implemented low-value features, but the high-impact and high-priority hard stuff was not done because some of your most experienced folks were helping the less-experienxed people write effective unit tests or whatever.