Some thoughts below - this is not to recruit or scare anyone thinking about being Program Managers, but just to give some insights after the experience with them in different environments and also without them where I had to be an acting PM.
OwnershipThis is big for me. If you don't have a Program Manager, you need to go outside of your comfort zone and try to find out the right answer yourself. You may not have the skill - or the time - to do this job well, but because there's no one to which you can outsource this work, you feel more vested in it. It's related to the startup culture I mentioned in the post Too much work, too little done.
PM technical expertisePM's role depends much on what product they are working on. Some products, like Microsoft Word, are way less technical than providing a low-level infrastructure technology like Amazon Web Services or Windows Azure. PMs' lack of technical expertise can bias some decisions and focus. Not to mention communication with customers that oftentimes could be more precise and more helpful if PMs were more involved with the technical details.
AgilitySo what I've seen happening quite frequently is that PMs were supposed to make decision for which they did not have enough info or background, so developers had to spend a substantial amount of time providing this info in order to get the PM's to decide and/or sign off on decisions. That was not very agile.
Customer FocusFor products more customer oriented, like Microsoft Word mentioned above, PMs were invaluable as they did all the work to understand customer's needs, talked to customers directly, supervised focus' groups, come up with the right features from the customer's perspective. That is something I really missed at places where we did not have Program Managers and developers did not have the skills or time to do the work.
The problem is that, for the infrastructure work, though, PMs were not fulfilling the same role. We need more understanding of the customer's scenarios to provide the right guidance. That simply did not matter and many decisions were made based on guesses.