A Berserk build is specialized for spike damage, not sustained damage. Think Catastrophes. You want to end the Rage phase (the damage check) ASAP. After that you just need to generate Burst and ready your ability cooldowns for the next Rage phase. Titania can even out the Burst generation to get in a second Burst if needed, but that's sort of unique to Wind.
Rapid Burst builds are an alternative focused more on 5-6T damage. In Wind builds, the Provisional Forest cooldown can actually end up being a bottleneck. It's not clear whether the extra elemental ATK from one weapon is worth forgoing the useful skill on the other weapon that further improves Burst damage (together with extra HP). It's probably situational.
Is it worth farming for both a weapon for Morgan and one for Shingen? Maybe if you don't have enough spike damage for some fights with Shingen (and don't need the EX slot), or enough sustained damage in other fights with Morgan. Morgan isn't terribly flexible; she can only Berserk once every 8T, and triple attack only once every 10T. If you need optimized spike damage more often than that, or you want to rely on Gaia's defensive skills, then Shingen will be the better option.
As for the debate over whether Regalia are difficult to farm, imagine you were a Rank 40 user that could barely nick the Catastrophes and you were relying primarily on (similarly weak) friends, who probably only join once the boss is half dead. Spawning raids that might not be cleared could actually be a waste of time and resources.