-Spd Alm and Windsweep

Windsweep prevents you from doubling any enemy when you initiate the battle (keep in mind, you can double when you counterattack). In exchange, Windsweep makes it so that - if you are faster as the opponent - sword, lance, axe and bow users cannot counterattack you at all. For example, if Alm (30 speed) with the Phantom Speed seal (+5 for skill calc) attacks a +spd Ike (34 speed), Ike will not be able to respond. But if you were using a -spd Alm (27+5) or forgot the seal (30), Ike would counterattack normally.

You have to decide youself. But maybe I can help you understand. When Alm starts attacking he will NEVER double. That's a downside compensated by the fact that if you´re faster than the enemy this one won't be able to counter attack. All this when Alm iniciates the attack. If Alm is attacked none of this will happen. You basicly want this for a unit with decent speed (like Alm). Not too high, not too low, just a good amount of it.
Why? Because if Alm were so fast the he could double enemies constantly the skill would just make him not double, which is a bad thing. The enemy won't counter attack but you won't strike twice. Although when the slow enemy attack you will counter attack twice,
If Alm were slower than the enemy, this one could always counter attack.
However, if Alm has between 1 and 4 more speed points than the enemy, Alm would not double either way (so the bad part of the skill doesn't matter) but the enemy would not counter attack. I hope this is of use.