I agree with you in the context of ads, but I do wonder if they could monetise these communities in a more high-value way which plays into their primary purpose.
For instance, I am a member of /r/synthesizers, but all trading/buying happens on eBay or reverb.com. Can reddit provide the way for this community to transact in a more high-value/trusted way? Maybe I'm missing something and that is a terrible business.