Well what I was thinking of was Beasts as a "detachment" for another Daemon unit. I would not consider using the Beasts on their own except to take down light skirmishers, small regiments of archers etc, or to tie up swarms (although I admit the very high points cost of the Beasts would seem to be a waste in this role).
Beasts get the Regen save against the majority of attacks, and if an opponent targets them with flame-based attacks he isn't targetting my Great Unclean One upgraded to Regenerate or my large units of Plaguebearers led by Heralds (which allow the entire unit to Regenerate). Fire-based attacks are not hugely common. And I'd still get Ward saves against these, since you can CHOOSE either Regen or Ward saves.
If a canny opponent started bringing Bright wizards or flaming weapons I'd simply replace the Beasts with Fiends for a battle or two, or simply use the Beasts as multiple-wound diversions (I'd rather lose 2 Beasts than my Great Unclean One).
So the Beasts get an average of 4 attacks each, plus an automatic Stomp attack each, they cause Fear, they get a 4+ Regen save, they have a ton of wounds... If I hit a large block of Clanrats or Swordsmen in the flank with 3 Beasts while 20 Plaguebearers and a Herald are already locked in combat, the Beasts would be shovelling out between 6 and 21 attacks plus up to 3 Stomps.
Finally, if I have a regiment of 3 Beasts and they undergo, say, 2 particularly punishing turns of shooting from Dark Elves with repeaters, presume they take 15 hits per turn, of which 7 Wound, of which 4 Regenerate... that's one dead Beast and one wounded Beast in 2 turns. Once the Beasts charge the archers, yeah one will die, but the other gets around 4 hits back plus a Stomp, all wounding on a 3+... I'm not saying the Beast would win, and it's not as impressive as you'd expect from a 100-point model, but it allows my 670-point GUO to lead his minions straight up the centre, laying waste to everything that gets in his path.