It depends what your alternatives are. This thread will fill up with comments saying start-ups won't pay as well as Google et al, but that's only relavent if you can get hired by Google (and you want to be). Comments that state "start-up wages are less than the best paying tech jobs" aren't very useful. If you can't get a job at Google then you need to compare a start-up role with a job you can get. If that's a junio…
But you are on spot on. 99% of the time startups wont make you rich and their biggest attraction by far is working on things you are care about and are interested in which you know you wont get at a BigCo (unless you are very very lucky to be working a hot new product blessed by someone high up). So if "interesting" things are what you want to work on why not just do a side project. This wont make you rich but atleast puts you on the path of producing "small" things to build your brand. Prioritizing 5-10 hours a week is not hard. Key is prioritizing.