Yay! I completed all the 7 levels and had a lot of fun!
Solid idea, and well excecuted. It feels nice to have the lasers as kind of a progression meter but at the same time something that can help me out. It was both fun and challenging when I regain a few lasers and then I loose them again.
To increase depth it might work adding fast paced strategic elements, like: oh man I really really want to defend this laser because it zaps so many enemies. Perhaps if the enemy spawners where more predictable, nodes had different repair speeds, or something :)