What do pruning nodes contribute to the network?
From release notes https://bitcoin.org/en/release/v0.11.0 :


  For now, block pruning disables block relay. In the future, nodes with block pruning will at a minimum relay “new” blocks, meaning blocks that extend their active chain.


In the current version pruning nodes don't advertise themselves as having any blocks, so they don't upload blocks. So that explains your big reduction in upstream data. A future update will re-enable block relay in such a way that your nodes can efficiently let its peers know which blocks it owns and which it doesn't.

Yes your pruning node will do full validation and it will service lightweight nodes.