While you cannot activate one while inside your vehicles, its pretty simple to fly forward, deploy, and activate during the shooting phase instead of shooting. The Webway portal is a 3" dia dome, that must be placed in base contact with it's bearer. Once placed, any non-vehicle units can use the Webway Portal. It acts like a board edge for bringing in reserves, and deepstriking and outflanking units can choose to come in via the webway. It's indestructible and Impassible.
So how far forward can you get activate your Webway Portal. Transport flies forward 12" + Deploy 3" + Activate 3". This puts your board edge possibly 18" forward and only 6" away from the enemy line.
Since only non-vehicle units can use the Webway Portal, lets look at units that make the most efficient use of our portals.
Wyches. We charge out of the webway with haywire grenades. 15 wyches charging into a chimera line should be able to disable pretty much the entire thing. Wyches move 6" fleet forward up to 6" and then assault 6". Yes that means it's possible to reach the opposite board edge.
Hellions and Beastmasters. Emerging from the Webway your reach is up to 24" in assault
Scourges and Reavers are able to move 12" and fire their 18" guns giving you up to a 30" death range. That means if your portal is dead center in the board, you can reach everywhere but the last 4 1/2" of a board edge. Correctly place your two Webway Portals on the board first turn, and there is nothing you cannot get to. Reavers turbo-boosting can absolutely reach anything out of a webway with it's bladvanes and caltrops.
Trueborn, Incubi etc can all move out of a webway, with easy to calculate ranges and effectiveness.
Cronos Parasite Engine and Talos Pain Engine. Want to get these units up quickly into the fray before heavy weapons take them down? The Webway allows an excellent means of getting to a forward position where you can do something effective. Who wouldnt cry as 3 talos come ripping through a webway into the core of your troops.
Using webway portals to quickly get your troops into forward positions for objective grabbing etc, and will be rather shocking for most opponents. Using three you could effectively create an 11" wall of them blocking objectives that you can use as a table edge for your reserves. They will be hard to block in and effectively disable, and multiple webways nearly impossible (even then you have a board edge). Dark Eldar strike hard and fast, the use of webways greatly enhance this, creating a very flexible and dangerous fast army that is able to reach anywhere on the tabletop they choose. Devil Nacht has truly arrived.