I actually took an old Endymion into Brit waters last night to test out reworked reinforcement zones. Hit a TBrig, and he got... a Belle Poule, if memory serves. The BP didn't really do anything, but the player was kinda slow on the uptake and kept trying to run instead of hooking up with the Belle Poule.
Eventually, he got other players in the fight, became something like a 10v1, and I kamikaze'd him to take out the TBrig, and was sunk after a while in the process.
My take-away? Reinforcements are only as useful as the player who calls them. If the player does not care to stick by them, they're going to be useless. Other players who come in and who can guide the player will give better guidance in real time, and say something like, "Hey, sail towards us and we can protect you."
I'd say they work as intended. They can scare off a player (or not, if the player doesn't care about the ship), but not guarantee that the invader is sunk for daring to sail into a protection zone.