A long while back, I ranted about how people want to pick the stables, lumber mill, and mine, and why that's a stupid idea.
Having been trying to gear up my druid, and watching us get smashed over and over again in Arathi Basin, I hear the same thing over and over again: "Guys, just defend three!"
This is fail. Epic fail. The only time this works is if the other team is an uncoordinated pile of boobs that send only two or three people into your 5 person defense each time. (What up, Alliance teams!)
Generally, the horde team is better coordinated and better equipped. When this happens, they will coordinate a 10 man zerg against your defense. After the cap, they will quickly swing back to defense. Assuming any of the alliance can even think fast enough to counter-strike at a non-zerged node.
So, what do you do instead of the 5-at-each-node-failure-mode-of-defense?
You either coordinate an unbalanced defense, or you place heavy pressure on a fourth node. However, both of these require people to be on the same page. An unbalanced defense is where you push 2-3 to each node and swing the remaining folks from node to node depending on where the zerg is heading. This has the drawback that if the horde team is smart, they'll push 7 to one node, and 7 to another, and swamp whoever wasn't reinforced.
Pressuring a fourth node is usually a better option. Leave 3 at each node, and push 6 to a fourth node. If they don't succeed, and die... when they start to respawn, push those that were defending onto that node. Versus a PUG horde team, that'll usually keep them busy enough to not coordinate a counter-strike.
Of course, if all of your alliance is poorly geared, the horde will just destroy you. This seems to be the biggest problem with winning recently... a bunch of purple equipped horde versus the craftable full blue sets of the alliance. Ouch.