I use Audiobus 2 and AUM together, they're a perfect combo as far as I'm concerned.
I couldn't imagine myself without Audiobus, if anything because it allows me to route multiple audio channels from my USB audio interface to plug in mics and instruments, or route AUM channels to Group The Loop (best looping app!), which AUM can't do by itself. And it has state-saving.
As I just found out, Remote is also an excellent tool for global transport control.
And let's not forget that many of us still use older devices that aren't AUv3 compatible and never will be. I have no intention to upgrade to a fancier iPad when I can already do most things on my current one with Audiobus 2 & co.
As I see it, the iOS environment benefits from having many different ways of doing something by combining apps in a modular way, rather than only offering one standard go-to app that does everything between heaven and earth. It allows for workflow flexibility. The number one reason why I hate composing on a laptop is because it forces me into a workflow that doesn't suit me, whereas on the iPad I can just build one up that works for me and that grows with me. Audiobus is a huge part of that.
My life and music making have improved due to Audiobus.
All I need.
Well, Audiobus surely is not in the same boat as AudioCopy. I currently use Audiobus all of the time. AudioCopy was essential for me like a couple of years ago when I first started checking out iOS music apps, but I can't remember the last time I actually used that app. I much prefer using AudioShare's workflow to using AudioCopy.