Why is Dark Soul a better buff for Imp Swarm than the metagem proc?
Dark Soul doesn't benefit the full duration of Imp Swarm (Imps CANNOT get all 10 casts out per Dark Soul).
Dark Soul is additive, the metagem proc is multiplicative. Assuming you get the full benefit of both, the metagem proc should be 30% extra Imp Swarms, while Dark Soul is (at 10k Mastery) only ~23% extra damage.
So, then why is it so important to line it up with Dark Soul? Intuitively, I'd think it's best to just line it up with any major buff (Dark Soul, metagem proc, Lust) to maximize the buff uptime your imps get.
Obviously the first Imp Swarm gets lined up with just about everything, providing you with the DF to make it through your first Dark Soul and providing some nice Soul Fire procs.
Later during the fight, you should typically have enough DF to make it through Dark Soul, even spamming Soulfire. You can also save MC procs and burn them almost like their own resource, so you shouldn't need the MC procs from Imp Swarm to maximize Dark Soul.