I guess my question would be how commonly are they used?

I can see using a 4x5 back on an 8x10 to Polaroid proof it, but I've got enough to carry with a bigger camera that I don't want to add more stuff! (Might be less of an issue for a 5x7 with a 4x5 back).

For the record, I've never used a reducing back, nor plan to use one, but that's just me.