I was thinking of the 21700s as well. Recently, got a tube which looks to accept them. It also has an inner sleeve for 18650s. The biggest debate for me would be having a need to get a new charger, maybe. Topside lite is also geared toward 21700s. Ah well, something I can think about.
Not so much for me as well. What I'm looking at though is probably getting a few tube mods that'll be designed for 21700s. Might also get a mechanical squonker, or couple which use them. In that sense I would be adding to what I have that I know works, only some taking different form batteries.
Yes, I know lots do include ways to use 18650s and that's good too. Still why not use what a device was designed to use? That seems reasonable enough for me consider buying different batteries. May get a set for the tube I have just as a testing run. I'll see, not quite full on determined myself to do anything yet.