I don't value spies so highly. Even if he does open the gates, it doesn't do much. I still need to create additional breaches in the wall, because squeezing your entire army through the gate only, is suicide, especially if it is made of stone and has oil.

I find that if you open the gates you are supposed to have the element of surprise on your side, doesn't count much in RTW though, due to above mentioned reasons and because the AI still places his troops right infront of the gate. So it only offers disadvantages instead of any benefits.