I have an extension on my good bass but I am super-conflicted about having one. It's so satisfying to play those low notes, and I'm usually the only one in my section who can reach them. Also, pieces like the Eroica are a bit easier when you don't have to cross strings in the middle of a run to reach a transposed Eb.
On the other hand, the extension is hard to play on for fingered notes, it takes special strings, and it adds weight and cost to the instrument. And there's no telling what effect it has on the tone of my instrument (I've never taken the extension off to see, because it's a hassle).
If I was sure the rest of my section would contain at least one or 2 extensions, or that I wouldn't get called up by a better orchestra, I'd probably take it off and forget it. Of course, in a perfect world, I'd just have Seth make me a second, smaller bass, and I'd leave the extension off.
One more anecdote about 5 strings - I lived in Denmark for a year and rented a bass there. It was an old Benedikt Lang 5 string that had been converted down to 4 strings. It even still had the 5th machine on the scroll. It was very reasonably sized, sounded nice, and played great. The luthier I had rented it from told me that the bass had been a lousy 5 string, but sounded much better when they took off the 5th string and adjusted the bridge & nut to match.