Putting aside the problem of ads for customers, ads also present a critical limitation to the businesses relying on them to make money: they treat all customers the same, and therefore do not maximize the potential value that your best users could bring you. Whether you’re an avid, passionate user of a service or an infrequent, casual user, the ad…
It’s hard to comment in the abstract. To your first question (development vs. cusp), I think it comes down to “can you build it?”
In the research phase, the answer to is usually “I don’t know.” The cusp is “probably.” The development phase is almost “yes.”
One important distinction to think about in the context of venture capital is the distinction between “research” and “development.” In general, a company can be in one of three states — it can be solidly in the research phase, solidly in the development phase, or just on the cusp between them.