it depends strongly on why you want to write open source. if you like the idea of putting source code out into the world for other people to use and benefit fro

it depends strongly on why you want to write open source. if you like the idea of putting source code out into the world for other people to use and benefit from then go ahead and use whatever mix of open source and proprietary code you like, just be up front that that's what you are doing. if you want to promise open source software simply to attract the mindshare and users who habitually ignore anything that isn't open source, trying to capture financial value may well be infeasible unless some rare confluence of stars lines up for you. the key is in the word "capture" - capturing the value implies making sure it goes to you rather than to someone else, and that means imposing restrictions that will simply piss those same users off.