You very rarely need those in folk music (or any other sort). And when you do, the double holes are no easier than halfholing and give you less control over the intonation - if you want those at all, chances are you will be playing music where D# and Eb are different.

Most of the recorders I use are either Renaissance types where the single hole is standard. I also use large sizes (tenor and greatbass) where it's more common to have a low C key but not a low C# one.

Getting a low C# on my sax is a pig - you need to push very hard on two of the biggest, clunkiest keys simultaneously with both little fingers. It obviously wasn't designed (back in 1922) with the expectation that you'd be doing it very often. Same goes for old simple-system clarinets; the "patent C sharp" mechanism came along quite late and my oldest clarinet doesn't have it.

I can think of only one commonly-played folk tune which has a sharp seventh at the bottom end of its range. Let's see if everyone else nominates the same one.

There is one truly appalling text used in British primary schools, "Abracadabra Recorder", which completely ignores the acoustic and ergonomic idiosyncrasies of the recorder and treats it as a surrogate keyboard. The authors throw in the low C# very early on, well before they've presented enough music to make the instrument interesting. The result is you're confronted with a physically very difficult manouevre with no discernible reward for it. That one pedagogical mistake must have had thousands of kids giving up on the spot.