What I've noticed with the Berserker Charge Bar is that some builds gain Charge ridiculously quickly (e.g. Storm Hatchets), while others cannot gain Charge at all (e.g. Wolfpack).
Since the Berserker gains 100% chance to crit when fully charged, and given how quickly the charge bar fills, this means that:
Builds which gain charge essentially have 100% critical chance. Almost
. Frenzy Mastery actually helps these builds because you reduce the duration where you don't have 100% crit (i.e. your down time), by increasing the duration where you do have 100% crit. It is subtle, I suppose.
Think of it this way, if you need 2s of attacking to get from no charge to full charge, then you would simply go like this: 2s of normal crit -> 5s of only crits -> 2s of normal crit -> 5s of only crits etc. etc. (where that 5s, could be 6.5, 7.0, 7.5 etc. etc.)
It is not completely useless, but relatively mild. (depending on builds, your build may even generate charge much, much quicker than in 2s)
As for other builds (i.e. the ones that don't generate charge with their primary attack skill), these builds tend to rely on other forms of generating charge (namely Storm Hatchet). Frenzy Mastery is particularly relevant for these builds, for obvious reasons. You spend less time generating charge, and more time hitting stuff. Simply put, you're more efficient.
Personally, though, I feel the Berserker charge bar trivializes DEX, and +Crit Chance. Since having it at full grants you a +100% Crit, it also means that +%Crit Damage can effectively be treated as +% Damage.
It is a unique aspect of the Berserker, and I'm not sure if lowering the Crit Chance that the Charge bar gives is a good way of fixing it, if it even needs fixing at all.
(P.S.: Also, Banner gives, like +40% Dodge on 15. DEX isn't very useful for Berserkers, apparently)
Probably a bit radical, but how about fusing Blood Hunger and Frenzy Mastery?
This is viable, and I feel there is a line of thought behind fusing these two skills specifically.
Frenzy Mastery may be considered a Charge gaining/retaining skill (technically, it holds your charge there longer), and Blood Hunger is a skill that consistently procs while you are in Frenzy mode (i.e. full charge).
Combining both skills would result in a skill that lets you stay in Frenzy mode longer, and continuously regenerate while in it.
Although, I do feel that Blood Hunger's proc condition is way too generic and applies too far across the board for the Berserker.