The idea behind the Reinforcement Bay is to keep the game from slowing to a crawl. One of the issues people have had with Renegade (and Renegade mods, by extension) is that the games rely heavily on the War Factory and the Refinery, to the point where any team that loses these two structures is considered crippled. Each game has their own answer to this problem; the Reinforcement Bay is AR's. The income provided will never be as good as the Refinery, and the vehicles provided will never make it a substitute for the War Factory. What it will do, however, is provide enough support for a team that has lost these two structures to hold a successful defense and possibly, if the team is smart enough, allow them to turn the tide of the battle.
I admit, I was initially skeptical of its inclusion, but after seeing how it plays out in a few rounds of testing, I think it's going to be a welcome addition.