Earlier quoted context omitted.
This is the wrong attitude - there is no "waste" of braintrust. Innovation doesnt work like that, it comes from all over by people working on what they want to work on. What's wrong with advertising startups? Some of the most valuable companies on this planet are advertising companies because it's an essential part of success for any business.
Most people in most startups are not "working on what they want to work on" they are working on what their boss wants them to work on. Heck, I would wager that >50% of startup employees are not even doing the work they think is most important for the success of the startup . They are doing the work their boss (who by definition is further removed from the facts) thinks is most important. Heck, I wouldn't be surprised…
How much freedom you have within a company, the power structure and the actual composition of the work is a completely different discussion.