On the defensive ward idea...

I don't see someone putting gateways right up to the lines, that would be asking for trouble and bottlenecking the troops, so they'd be a fair distance away. Set wards that would cause a very visible explosion, like a pillar of fire, or something in a bright color, and train your own channelers if they see it to bombard the area.

With the explosion, you have a chance of causing some damage immediately through the gateway, possibly killing the channeler and some of the troops. At the least your alerted and you can start bombarding.

A gateway would have to be open for at least a period of time to get reinforcements through. so for the troop movements the instantaneous effect of gateways is negated somewhat by the length they need to be kept open.

Unless it is to slip in one or two channelers at a time.

From the defensive battles we've seen. Caemlyn and Illian and Shadar Logoth, it is important to keep moving. Even on the assault, such as at the Cleansing. Those were very fluid.

So for normal troops the argument can be made that they wouldn't use gateways as much during battle, but before for staging and later as retreats. During would be for just the channelers, who are the heavy artillery, and giving them strategic positioning.

One channeler behind enemy lines I think would be worst then a thousand soldiers.