Exactly, there should be no such thing as deep-strike PvP with no consequences. Having territory should mean something. If you want to deep-strike, organize a sizable fleet to do so.
4 man gank squads should not be able to roam freely in enemy territory picking off lone noobs wandering around.
I think some kind of BR limitation to keep the fights relatively fair while providing a bit of home-field advantage would do away with join timers.
If the battle happens outside of any reinforcement zone, then the battle can only be joined if the side you wish to join has, say a greater than 10% BR disadvantage. If inside a friendly reinforcement zone, then your side gets, lets say, up to a 15% BR advantage before the battle is closed to new entrants. Just numbers off the top of my head, the details of the mechanic might need some tweaking.
It could allow OW battles to grow organically depending on local player activity while still avoiding the "surprise, here's 20 of my friends" situations and doing away with the very artificial feeling timers.
Just an idea to consider. I see a lot of people getting frustrated when they try to help out some friends and constantly get in range of the instance 30 seconds after the battle closes. It doesn't promote a natural-feeling or fair open-world PvP experience at all.