Is using Regen a bad practice for end game contents?

[Amilherix]

Its better for longer fights I would say (Tank Perspective)

I would say one thing I see a lot of Healers not doing is refreshing stoneskin during fights, 90% of healers are only using it as a pre buff not refreshing it during fights.

That's because of it's 3 second cast timer. Also, it is 18%. Assuming 5000 HP that would be 900 damage prevented. I am assuming it is 399 mana at level 50 (based on the websites I found), that can be pretty mana intensive in a fight. I haven't played WHM, bu from my observations, WHMs that prioritize to keep this up go OOM faster than those that use other tools, like Regen. WHMs could buff up the group with Stoneskin before the pull to avoid that spikey damage during initial threat grab, then could begin their normal healing rotations.
Due to my observations, I would have to disagree with you wanting stoneskin during fights as a priority.