I think I've finally figured out why an airline must have adequate reserves even when it's cheaper to just INC the crap out of any trip they need covered.

I was looking at timing out (duty day) last week, and we still had about 2 hours before the inbound we were taking out was going to arrive (in base). I called to see if they planned to replace me.

Talking to them I felt like if they had a reserve they would have used them, but they couldn't warrant enticing a 15 hour INC guy to come out since there was still a small chance it would work out with me.

Ended up cancelling.