What are the positives and negatives of leaving a lightning payment channel open? How am I contributing to the lightning network and what are the risks if any?
In Electrum? I think keeping it open will not contribute to the network since Electrum-made channels (trampoline or not) don't route payments (CMIIAW).As nc50lc has said, channels founded using Electrum cannot be used for payment routing. All channels are private so they are not advertised across the network and cannot be found using Lightning explorers (ex. 1ml.com).
I think future versions of electrum should support the ability to route payments via LN. Without this feature, I can see a lot of LN nodes closing LN channels with electrum users if they do not use many transactions, especially if the electrum user has sent payments to/via the LN channel, meaning the LN node is tying up capacity. 
