Salted Earth: 780n pot/min
DA DP: 500n pot/min
DA C+S: 450 pot/min
Reprisal: 420 pot/min*
Plunge: 400 pot/min
DP: 300n pot/min
LB: 240 pot/min*
*These two skills are parry dependent. The listed values show the maximum value for reprisal and the minimum for low blow. Low Blow may have a higher priority under the effect of DD.
SS: +0.5 DA
C+S: +0.5 DA
DA DP: -1.5 DA
DA C+S: -1 DA
DP: -0.5 DA
DASE: -1 DA
Grit: -1.5 DA
Darkside: -0.5 DA
Although I said that MP efficiency doesn't really matter, outside of BP and BW, you only get +4.5 DA worth of MP per minute (24 gcds = 8 SS + 1 C+S). Some of this has to go into maintaining Darkside.
In single target, you're only using DA on C+S, so you're only obliged to effectively spend 2.5 DA per minute. In multiple target situations, however, if you want to use DA on both C+S and DP, it costs you a whopping 4.5 DA per minute. This forces you to maintain your Darkside upkeep with only BP and BW. When you add other expenditures (AD, inability to use BP in tank stance, etc.) you can see how it is impractical to use DA on both.
Although DA DP is a dps gain at 2+ mobs, a single use is only 57% and 86% the potency of DA C+S for the same cost (1.5 DA) at 2 and 3 mobs respectively. Fight-specific considerations aside, you should not use DA DP at the expense of DA C+S when attacking 2-3 mobs. At 4+ mobs, the potency of a single use DA DP exceeds that of DA C+S, and the skill actually becomes efficient. At this point you can use a regular C+S, returning your obligatory MP costs down to 3.5 DA per minute (same as single target).