They don't necessarily have to make their revenue all from ads.
They can provide a polished, stable version of Android that is in many ways better than the original and provide support to the phone manufacturers (perhaps more cheaply than Google?), directly getting a cut from handset sales.
That seems to be the obvious value in this company. Phone makers have demonstrated time and again how badly they manage their own Android distros, and how much they see them as a cost-centre rather than an opportunity to better position themselves in the market.
Given the state of current MBAThink, why wouldn't they want to outsource the heavy lifting of distro management to a company that not only does it well, but does it better than their competitors? If CM play their cards right, they could start a bidding war, or at very least, make sure that their's is a seller's market.
I especially like this idea because it upsets the playing field a little. Cyanogenmod can be ported at low cost to a number of generic platforms, allowing cheap(er) phone makers to gussy up their product without much effort. So to the consumer, there won't be much to choose between an SGS4 and a KungPaoDuk Delightra XXS Happy Screen. (Visually, at least.)