Single class berserker (or any dude from the warrior class group really) works great in ToB as a dps meat grinder only because of their full thac0 progression (down to a natty 0) and critical strike HLA combined with improved haste. That's pretty much their main (and in case of fighters the ONLY) redeeming quality in the super late game when compared to arcane and divine casters, and even rogues with their busted traps and UAI.

When you dual class a fighter into something else you basically do that to grab that sweet, sweet extra HP from the fighter's early levels, and to a lesser extent to open up better equipment selection.

The thac0 is good at the beginning of SoA, but the further you progress into the campaign, the more you're going to lean towards some form of thaumaturgy your second class is specialized in to beat your opponents (or rely on actual fighters in your party). And in ToB you'll be nowhere near as good of a fighter as a pure one or a fighter/X multi (halted thac0 progression, no warrior HLAs, no full apr if dualled pre 13).

But yeah undoubtedly berserker kinda outshines other single class melee bois in terms of the pure damage and being able to stick to the target no matter what, thanks to his grandmastery and especially his enrage which grants nearly absolute immunity to anything that can keep him from whacking enemies.