One mile <> one mile, you have to watch for speed on final. Also scenarios are different depending on a/c types - as Spitoon says, usually you're happy with the departing one getting its wheels up, so surely you have to judge how long a given a/c type takes to start its roll, how long will it go down the runway etc. Also, if the arriving a/c is slower (e.g. 737 departing, ATR arriving), a possible go around scenario gets easier, as it has small chance of catching up with the preceeding.
And, if we're talking about speeds, then wind comes into consideration. I did line up and depart an a/c with 767 on 2 miles, but it was doing 80kts GS due to headwind
