Not all that useful if you can't transport food and spare weapons and big war machines etc... sure you can send tons of men from one spot to another. But, as DREWRULESWOT said, it more useful for a raid than practical army needs that they will be unable to meet easily.

Also consider how many times have we seen 100 damane/sul'dam paires in one place, even Ituralde only encountered a few pairs at a time. And how many women have the strength to open Gateways 6 feet by 10 feet tall to allow for horse/grolm/torm riders through? Seems to me that there are only a handful of channelers who possess enough strength/Talent to generate these gateways. More likely by far that there would only be 5-10 women with any given group of Seanchan with maybe a couple strong enough to make a gateway big enough for more than a single man to pass through at once. Even if they had women strong enough to open gateways large enough for carts and mounted soldiers they would exhaust themselves to the point of uselessness quickly.

Take Perrin and his Asha'man for example. They where moving his forces, but after only a few weeks of moving a relatively tiny force (some 5000-odd people) they were so exhausted they could hardly Channel. Grady and Neald are fairly strong, but neither of them could do much after a day of holding the Gateways open for the whole force to Travel through.