I love having a 5 when I know I'm either going to be transposing on the fly a lot, or there's any chance I'm going to be playing with horns.
I often played in situations where I'd learn a chart written in one key, and then, depending on who happened to be singing, we'd perform in a different one. Having the 5 gave me a bigger box to play in before I'd get stuck finding a new fingering - handy on more technical tunes.
Since horns often play in flat keys, I also really enjoyed having the root notes for keys like Eb and Db on the bottom string, or the fifth of keys like Gb or Ab available down there. Great for orchestra keys.