It really depends on your threat modeling and what you're concerned about. I agree, dual booting isn't ideal, but also, "dual booting Qubes and Ubuntu for gaming" cannot be any worse than "simply running everything on Ubuntu," as long as you don't believe Qubes is impervious to anything nasty in that configuration.

The main storage partitions are encrypted for Qubes (... or had better be, I guess you could avoid that, but why?), so the dual booting attack path would have to be through the boot partition, load something that can then compromise the install. It's a fairly specific sort of attack that, if someone's coming after you with that, it's probably a question of "How and when you're screwed, not if." But for general users, I think dual booting is an acceptable compromise. Just don't do much of anything in the other install!

I dual boot my laptop. There are a handful of things that are far easier to do in Ubuntu than Qubes (movies on a long flight, run a Windows VM to run particular software to talk to cars, and Minecraft for LAN parties). I'm aware of the risks, and don't consider them to be enough to remove the ability to do a few other things on one machine. I try not to have too large of piles of single purpose computers these days...