I have enough problem solo keeping RHgy and East and West tower (if I stay middle; some rogue or druid sneaks in and cap the towers; if I stay in a tower I loose RH or West...) so I would say indeed having 15 in a single place would be a disadvantage - unless you can actually get the teammates to listen (which I can't on alliance side - you can see my recent matches on my stream)

What I end up doing now is stay with my 5 in one of the tower; so at least we get as much, but it happens all too often that I can't get allies to stay in the other one... and we often loose it... and also sometimes loose RHgy too... xp in av is great but it can be frustrating...