A copyleft license is actually more conductive to Open Core development because it allows the copyright holder to reserve the right to create proprietary addons while shutting the community out of that option [1]. Not trying to insinuate that Light Table specifically is going to do this, but the option is still available.

[1] http://ebb.org/bkuhn/blog/2009/10/16/open-core-shareware.htm...