No. Just no.

Seriously, this game doesnt need to be even less punishing to people who mess up mechanics. We dont need even more hand holding, specially not in content thats supposed to be a challenge.
Is it frustrating when people die to simple mechanics? Yes, yes it is. Frustrating and annoying as hell. But this isnt going to lead to them learning to do those mechanics well - this is teaching them that hey, it doesnt matter if they mess it up, they'll get raised straight away! Oh, we're not in "practice mode" anymore? Well, we got two healers, a summoner and a RDM! One of those can just raise the dragoon who didnt bother to move because thats what they learned during practice!

Sorry... no. It doesnt enable "efficient practice", it would only give people a wrong idea of how the fight works. If they cant do the mechanics (and/or keep everyone alive, when being a healer) they dont deserve to clear the fight under any circumstances. Not even without loot.
The issue that people dont manage to clear those fights with pugs is that a lot of people who are pugging this content actually shouldnt, at least not until they get a better grasp of their job and how those fights work. "Practice mode" will do nothing to fix those issues, because those people who arent good enough to clear this without wiping 50 times wont bother with it - we have Stone, Sky, Sea for people to practice their rotations and check wether or not their damage output is good enough. Do you believe that those 4k-BLMs went over their and parcticed? (If they did, they surely didnt do the decent thing of not subjecting other people to their subpar-performance)
I'd also like to point out that you mentioned something about the books - so... would a practice-mode-kill reward those? Or am I missunderstanding something...?

Bottomline either way:
This would accomplish nothing - at least nothing good.
Those people who need more practice and are dragging down PF-clear-groups wont bother with it, because they're not self-aware and reflective enough to realise that they would need more practice.
And those who'd genuily try to use would get a wrong idea of those fights and potentially learn harmful "strategies" or the mechanics incorrectly because "Hey, it doesnt matter, I'll get a raise straight away!"

As far as SE is concerned the way Savage-content is meant to be played is in a static - or at least in a group that communicates well with each other. While pugging surely is possible, its not the intended way so...