Board Thread:Battle Nations Articles/@comment-174.1.0.88-20130422234000/@comment-5007614-20130422235214

This best details what SP, skill points, are: Promotions. Boosts are worth it if you are focused on promoting units and plan on battling during the duration of the boost.

If you're at the level of fighting the Ancient Construct, I think it's definitely worthwhile to spend some nanos on the 2x boost for the fight, then moving to the Boar and Raptor fields and clearing them out. During "Double SP Weekends", the boosts stack and you get 4x SP!