Becoming A Protectorate

[TinCow]

I have seen plenty of posts about getting other factions to be your protectorate, but I haven't seen much about becoming one yourself. Last night in my VH/VH Dacia game I was at war with both Gaul and Germania at the same time. I was frantic because I only had one army capable of defeating large enemy stacks and I couldn't defend both borders at once. The situation was looking grim, with one of my cities recently lost to Germany, with another beseiged by Germany and yet a third beseiged by Gaul. I sent a diplomat to try and negotiate a peace with the Gauls, hoping for even a turn or two before they attacked again to allow me to defeat Germany and transfer my army back west. Surprisingly, the Gauls offered me over 2000 dinarii tribute for 5 turns on the condition that I become a protectorate. I sat back and thought for a while... and finally accepted. I am now Gaul's 'ally' with trade rights and military access for both of us on both our lands. I can easily afford to pump out a couple more armies now and I am certain I can push Germany back. Eventually I will break the Protectorate and pursue Gaul some more.

So... what penalties am I suffering from being Gaul's protectorate? Other than them having military access to my lands, I can't see ANYTHING bad about this situation. What am I missing?