I once tried to create a 1.5mil point defence wall, but I noticed it wasn't really practical because you cant target weapons with point defences & focus your fire in one spot, even when you set them to "attack only" mode.
It was pretty convenient though; because a single one fits in a 1x1 square, you can fit an entire crew bunk, a power storage, and two point defences behind a shield arc.
Also you for some reason can't control point defenses with Direct Control mode. (The option for choosing a direct control binding isn't present when you select a point defense system and click on the three lines...)