But we aren't arguing, we're all agreeing! Discussion of regeneration is like a fine wine, it gets better with age (except that wines don't actually get better with age, but I digress).
I often like to think about regeneration not as +20 hp over the course of a battle, but rather about the rate of incoming damage vs regeneration over the same time period. If your defenses are high enough that fighting a pack of yaks results in 10 damage a turn, then regeneration reduces that incoming damage by 10%. After you kill one of the yaks, your damage per turn drops to 6, and now regeneration is reducing that damage by ~17%. This is usually more easy to do in games without huge variance, as crawl tends to be 0 damage, 0 damage, 0 damage, 3 damage, 26 damage, but it still applies as long as you aren't sitting at 100% health and regeneration is having an effect. So regeneration is more useful on heavily armored characters, than a high EV low AC character. You want to have been hit for some damage, but not much.
The higher your defenses are, the larger the chunk of incoming damage that regeneration will be able to negate. It's a bit of a tautology to say that some effect works best on high defenses, high spellpower, high hp characters, but that's the case with regen. This isn't meant to imply that you shouldn't learn regeneration if you have high EV, it works well for anyone really. The only time I would say you could possibly skip it is on very very low int characters - I've been unimpressed with it on centaurs with 11 int*, but that's fairly extreme.
*That game:
http://dobrazupa.org/morgue/tasonir/mor ... 104317.txt 10 charms, 14 necro, regeneration still had only 4# spellpower. 6# is usually where it starts to be extremely powerful, 5# is okay, <=4 is somewhat weak. YMMV, these are just guidelines, but they're approximately correct. I still cast it 304 times that game anyways
No matter how low your int is, regeneration amulets/randarts are always worth it.