Damage from weapon mastery is added to the enhanced damage from berserk and all the other sources like off-weapon ed (fortitude) and strength.
It is worth the same, not more not less. So 150ish ed from it is quite little compared to the 835ed of a level 20 fully synergized berserk.
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True, but the Critical Strike is calculated at the end, so 21% can be quite significant. If you have 958% ED (maxed Berserk, Mastery), then the 21% critical strike (from Mastery) will add as much damage-over-time as another 222% ED.
Basic comparison, assuming you deal 100 average weapon damage per swing and have no other sources of ED.
Maxed Berserk, 2 maxed synergies, 1 point mastery:
991.9 damage per hit
Maxed Berserk, 1 maxed synergy, maxed Weapon Mastery, 1 point in second synergy (requires exact same number of skill points):
1050.3 damage per hit
Keep in mind that Mastery also adds an AR bonus, to boot.
If you have *ANY* additional off-weapon sources of ED (Fortitude, Phoenix, Might Merc, Strength), that will further skew the comparison in favor of Mastery. If you have any deadly strike, that'll skew the comparison in favor of the synergies (although again, remember that Mastery also provides an AR boost). Also, a lot of +skills will skew the comparison in favor of the synergies. Still, for the vast majority of Berserk rigs, getting at least 10 points in a Weapon Mastery will offer a far greater benefit than putting those 10 points into synergies (and, thanks to the AR, I usually max mastery).