not getting warp strike drops DA's killing potential almost to zero imo. Unless your going AA DA
At Level 10 - L1 Swap - L4 Warp - L4 Spine: Mana (450+ 950 + 1000) = 2400 - Damage = (700 + 1150) = 1850
At Level 10 - L1 Swap - L1 Warp - L4 Spine - L2 Warp area: Mana (450 + 500 + 1000 + 1000) = 2950 - Damage = (250 + 500 + 1150) = 1900
More mana, same amount of damage and three picks instead of two. So you can pick up two levels of elusiveness or demon speed before level 10 instead of just one before level 10. Of course WA has a much longer cool down than WS, so if you're gunning for a WS - Spine - WS combo, you'll be better off with WS.
I'm still learning to utilize WA, but if you stick to creep waves, it's great. Any ability that you can see coming out (fireball, slam, etc) can be defeated using WA in a creep pile. And since it is instant and only targets yourself, it's pretty hard to screw up the timing. Big downside is it only works well if you're around enemy creeps.