I don't do any "sing along" in my performances. I just play and sing and give them what I want and what they seem to want.

I could spend time putting a medley of songs together, but not sure I want to spend time doing so.

My songs are all one man band, full arrangements. I don't want to get pigeonholed into an "old" act.

Thoughts? Is it worth it? Perhaps 20 minutes or so?

It never hurts to have extra things to go to in a show. You don't have to use it but if need be it's there. I have a sing along medley that I can make as long or short as I want on the fly because it's live. I don't sing in this case but I encourage the audience to join in. Works well and even if they don't sing they think you played it just for them.
No such thing as too much ammunition.

MEL

It's another tool in your toolbox. What's to say you can't use full OMB arrangements for a sing-a-long session?
Brown Eyed Girl, Buttercup, Sweet Caroline - these all get a rousing choral outburst wherever I work. There are many, many ways to do a sing-a-long.