piousflea:
Yes, Ghoul Rot is also really, really good. But you're missing multiple aspects of Bear Strength.
Consider the melee Beastmaster. He's still able to cast a single spell per turn, and a 2nd if he really has to, but what he wants to do is rush in and start punchin'. He casts Bear Strength on himself, maybe follows it with a Nullify on himself as well, and goes to town. He can do 6 damage dice every round in melee, for the low price of 0 mana. His large attack pool means that he has a good chance to KO creatures before they get to do anything, and his attacks are also unpredictable: they could do 12 damage in one hit! Given the way that Armour works, his attacks are much more threatening now to high-Armour creatures. And he's saving a lot of Mana every turn, so he can easily afford Dispels and Dissolves and other things that keep him on the offensive. Maybe he'll also bust out some Gauntlets and go up to 7 damage dice.
This doesn't even touch how scary (or appropriate) a Steelclaw Grizzly with Bear Strength is.
Now contrast the Flameblasting Warlock. Instead of casting Bear Strength, he got out an Elemental Wand and stuck his Flameblast on it. He's only doing 4 unavoidable dice of damage per round (though yes, with the possibility of burning), for the considerable cost of 5 mana/round. He has a range of 1, but because of Hindering, switching of Initiative, and the small size of the Arena, he will find it very difficult to kite the Beastmaster. If the Beastmaster puts on a Dragonscale Hauberk and the Warlock can't punch through enough Nullifies and Reverse Magics to make it Explode, he will become a very sad panda indeed.
Now contrast Ghoul Rot. For 1 more mana than Bear Strength, it does direct damage (so on average much more than from an equal-die melee attack), can also be Dispel-ed, but does not enhance a natural ability in any way. It is also a very predictable source of damage, so both Mages can plan around it.
And what do you do with your copious quantities of Beastmaster Mana? Summon more huge beaters from your Lair, of course!