Backswing Point

I know that if I set it to a really low value, units can have high attack speed but what is the disadvantage of it?

Will the animations look screwed up? I just want a pros and cons of doing this.

I don't know about that, in my test the backswing point didn't get in the way of the unit, the unit would attack the moment it could. The damage point, however, is capable of limiting a unit's attack rate.

Pro: Faster attack speed minimum since the delay between initiation and attack is smaller
Con: Looks weird if it's too low (e.g archers shooting arrows before pulling on the bow)

I see, thanks.

That's damage point, not backswing point.
A way to work around the animation weirdness is to give the unit a normal attack cooldown, but then give it a bonus attack speed ability; this way, the animation speed of attack animations is increased to match the unit's attack speed and the damage/backswing points are also scaled accordingly.

WC3 will scale the animation time (Backswing + Damagepoint) of a unit according to its attack speed and base attack time. Note that the engine cannot cope with incredibly high speeds (especially with low base attack times), and will cut off animations to a large extent as speed approaches ridiculous levels.

The internal counter WC3 uses for determing whether a unit may attack again does not actually include the backswing, however. Thus, it is advisable to keep the backswing to the defaults, or whatever looks aesthetically pleasing - you want them to synch up with the animations, ideally.

Note that the only downside to a backswing is that a unit will not automatically issue queued commands until the backswing is over. This means, for example, that on the strike that kills an enemy unit, the attacker must wait for the backswing time to expire before it will move along the order queue (attacking the next opponent, moving to a new destination, etc).

While you can manually interrupt the backswing by assigning an immediate order during the backswing period, the unit is prevented from starting a new attack until WC3's internal counter allows it. This means you can manually cancel the backswing to move, cast a spell, use an item, etc, but lowering the backswing will not increase attack speed.