Online de-dupe is fast becoming a requirement of large storage systems. There have been several discussions about the pros and cons of online vs offline vs why bother at all. I'm in the camp that says de-dupe is a *requirement* for some people, so please have it as an included option with the file system. It makes little sense to avoid doing it online with BTRFS, so stick it in at that point.
The arguments as to whether it suits my environment are best answered by me, so please let me make the decision as to whether I enable it on my disks.