Considering how two-handed weapons are fairly standard for strength-based builds which most barbarians are, Berserker has a good chance at being useful. On average it's an extra 4.5 damage, so it beats stuff like Flaming as far as pure damage goes.
By the way, will you be adding sources to the list?
Well, yes. but my argument is, this is a handbook about weapon abilities. Not a handbook about barbarians. It should be seen through the filter of "Generic Player" not "Player who likes two-handed swords". To the generic player, Berserker has two strikes. Very specific weapon type, and only usable while raging. True, a useful Weapon ability to the right player, but on a scale of 1 to 4, with 1 being it sucks, 1 being average, 3 being above average, and 4 being everyone could benefit from this, Berserker falls squarely in the Average catagory.
Now, that said, I cannot anticipate every possible combination of class/race/PrC/etc. but that's kinda while I have this discussion thread in hopes that people will point out things I'm missing. Lets say that if you are a halfling, long swords are two-handed swords to you, so that opens up a weird combo if you take aptitude and Exotic weapon masters or some shit. I'd like to hear about crap like that.
But unless you are willing to play a very narrow PC concept, it doesn't have much use. Some people are willing to play, but narrow focus means a decreased value in comparison to something like, oh, any of the Surge abilties from DM2. (Which, BTW, are BROKEN. So glad they updated them in MIC. 8,000 gp and I can hit you with 8d6 of extra damage with four different energy types? What the hell were they THINKING???)
Oh, and yes, everything will have sources because some DMs disallow certain books. I'm setting this up in the same format as The Spell Book.