The attacker does not have to waste time moving around. His troops are stationary, with each gateway opening directly in front of them. But at the other end, the gateways could be opening miles apart. Even if only 1 is spun at a time, the defender is at a disadvantage to try and counter them all. I think the gateway warding weave is just a rare skill, with maybe only Demandred among the Forsaken able to do it.

Also remember this skill is far more useful to the Shadow than the light, as it is purely a defense against attacking human armies. Shadowspawn can't Travel in any case.