Re: Cheap backing or substitute
I don't use mono on my fly reels for 2 reasons. First, mono deteriorates at least to the point where it becomes weaker over time. When I lived in Florida, we replaced the line on our spinning and trolling reels every year. I'm told that sunlight was the greatest enemy of mono, and it may not be as big of an issue when buried beneath flyline. Still, I'd rather not risk it.
Second, mono takes a "set" over time, and can become very curly. Anyone who has ever had to straighten a leader has seen this. Picture having to straighten 50 yards of mono backing.
Flyline is expensive, and does not get changed very often. While you see what the flyline looks like, and it's condition, you don't see the backing. Most of us don't get into the backing very often if ever and are unaware of it's condition. It is there for a reason, and had better be reliable when called upon. Stay with a product that you can rely on. It is worth the extra few bucks.
You can always tell a fly fisherman. You just can't tell him much.