Protoss Forge Fast Expand?

Recently I have been putting my forge at my nexus. Has anyone else been doing this? Does this leave you vulnerable to something? I don't get why more players don't do it.

Against Zerg.. You put your first pylon against your nexus. You then build your forge next to your nexus. (You have no wall at all) Then based on what you see you see you can either go nexus next or start putting up your wall.

If you face an early pool you are bullet proof. You already have a pylon in your main, you can drop a cannon. You do not lose any buildings. You do not lose any probes.

If you face anything else your nexus should be going down about the same time as the Zerg hatch if you probe block it for a little bit.

This seems to make a bit more sense than to throw up a pylon and forge at your natural then be unable to defend it and lose it to an early attack.