I would think that to shorten queue times is not the primary objective of this feature. Blizzard is going to change the VP reward system to allow players to get a week's worth of VP rewards in just one day. Imagine what would happen if all the tanks decided to spam it on the first day and then go MIA for the rest of the week. Call to Arms is a necessity.
Also a lot of semi-geared tanks and healers will be using the new ZA/ZG to cap their VP, probably in guildruns. So there's even few of them left for the ungeared, possibly unwashed dps masses trying to do the Tier 1 heroics.