I don’t know all the other apps listed here but I do know last I looked Nine was the only one that actually had Exchange ActiveSync support. ActiveSync is a licensed protocol so you won’t find it supported in a free app, open source or not, not without some alternative monetization that compromises your privacy. Nine has no ads and doesn’t run through their cloud. It also optionally isolates the security model to the app so remote wipe and other employer policies don’t apply to your OS. I also didn’t want the corporate address book integrated with my personal one, and it lets me keep that separate, too.