Level 8 farms are clearly superior to L7 farms. Although you need only three L8 agents to build a L7 farm, you need eight L8 agents to build a L8 farm. And you might not have 8 local L8 agents in your area. So you need to help each other level up fast. And even if you have them, you might not know each other. You might know each other, but not everybody might know everyone else. Or it might be hard to get them all together. That is where the Ingress twin Google Plus comes into the picture. Google Plus is the necessary alternate app to the Ingress experience. You can't have all the fun with only the Ingress app. Although it is possible to overdo Google Plus and make it get in the way of the Ingress experience. There is overkill.
L8 farms are so important that it is my claim the team that organizes more L8 farms in a given territory is going to dominate that territory. That one metric alone can be the deciding factor.
People who play the game are aware of the importance of L7 and L8 farms. And so there is a constant attempt to bring down the other team's L7 and L8 farms. There are top agents in New York City on both sides who specialize in bringing down L7 and L8 farms. The idea is to deprive the other team of high level ammo. There is a certain thrill to showing up to take down a farm. You usually end up having beer with those agents on the other side whose attempt you just foiled.
L7 farms are trickier. It is possible there are three local L8 agents in one location. The attacking agent might have to travel. But how much traveling can you do? There are some parts of the city that boast of near permanent L7 farms.