Constructive & Helpful!
This is still a bad use for the skill, Frostval Barbarian has an extremely high natural Haste, reaching the 50% cap completely on its own without the need for any Enhancements, but using Enrage reduces your Haste to *bellow 0%*, and Haste *can* go into negatives, using Enrage will make your AA and cooldowns work at bellow half normal speed while still dealing only 2x damage, ultimately you have a net negative DPS in using Enrage, while also making your buff stacks more difficult to maintain, it is objectively a bad skill.
With full fighter without using Enrage, you are looking at 366 damage per 1.5 seconds (using my level 88 numbers again), or ~242 damage per second. If I were to use enrage, I would do 2196 + 732 (or 2928) damage approximately every 4 seconds. Divide that by 4 and you have DPS of 732.
Silver Thaw/Maw/Jaw does literally nothing for FB without other people nearby. Winter's Depress has a barely noticeable effect. Frostval Gifts literally is only a heal as long as Enraged is not in effect. Neither of these skills increase your effective damage output, so it doesn't really matter if you use them or not. It does have AoE auto attacks, and you could argue using Spiral Carve could (potentially) make it a slightly more viable, general AoE class, or the use of Health Vamp could give it at least some form of sustain, but then you might as well use Vampire Lord or Abyssal Angel instead, which both have better natural sustain and better long term AoE damage potential. I would personally just stick to using FB for small mobs, supporting, or banking it if none of those are applicable.
It's not an objectively bad skill, it's just a skill with a heavy drawback, but also a solid gain. An objectively bad skill would be Chi Blast from Chunin, it's counterproductive in almost all situations. It deals about as much damage as Enrage, cannot crit, cannot miss, but also stuns you for 4 seconds, literally making it unusable outside of being 100% sure you can kill off all enemies hit. At least if Enrage doesn't necessarily kill everything, it still almost triples your damage output, assuming you keep using it.