Is there anything that makes Bear's Balance more than just situationally useful? Or do fear affects come up more than I think they do?
In addition, are there any other feats/abilities like that that auto-rage you under specific conditions? Such as damage or other status effects?
It is only situationally useful, but it is rather effective. A lot of spells have the fear descriptor, particularly if you are going up against evil casters. A lot of of evil outsiders have fear auras that can be crippling, and mummies in particular have an aura of fear that will leave you paralysed.
It won’t come up every fight, but if you have a bad will save and are expecting to fight undead and evil outsiders (who isn’t?) it can really save a party.
Plus entering a rage instead of giving into fear? Bonus style points.