PBEM constitutes 100% of my game play in CMBB and CMAK. I never went back to CMBO after the others in the series were released. For me it would be terminal to lose PBEM. An alternative form of asynchronous play would be acceptable but it would need to be there for me to use. The we-go system that these games have pioneered is the "hook" that got me addicted to these awesome games. I adore the replays I get back from my opponents. It is fantastic to be able to watch and replay the actions and incidents over and over again.
Watch, replay, enjoy, think, plot. That's what PBEM format gives me. Oh, that and the privilege of having made the aquaintance of many fine and knowledgeable gamers spanning the globe, in completely different time zones, that I could never, ever, have played against without the PBEM/asynchronous play option.