Those people who would rather have the spare posting time and freedom of being a humble poster do not have “Moderator” after their name. 
Some people (me, for one) have declined–at least at first–an offer of the Mod position based on its time-suck attributes, and the amount of time involved in moderating has undoubtably contributed to Mod burnout.
Gaudere is so modest and retiring. In my case, I was licking the monitor and salivating with eagerness, and when I was offered the moderator position, my response was “Thank you sir, may I have another?”
As far as your question “Has anyone voluntarily stepped down from the position of moderator?”, several people have done that. As far as stepping down as moderator and reverting to “regular poster” mode, that hasn’t really happened yet, but there’s no reason why it shouldn’t. E.g. JillGat took a break from moderator duties and then came back, but in the interval I don’t remember that she posted that much.
Why did I want to become a moderator? Part of it was the desire to help out at the SDMB since I spend a lot of time here anyway, part of it was because I thought that I could be a valuable contributor in setting the “tone” that I like to see at the message board, part of it was curiosity to know what happens “behind the scenes”, and part of it was the thrill of knowing that there is a certain status (and/or stigma) of having the title “moderator” under your screen name.
I think that what we’ve learned here is that the real reason moderators don’t post very much is that not very many threads are about them. Put the word “moderators” in the title, however, and they come out of the woodwork.
We do not.
Just what the hell did happen to Nickrz?
I don’t know about anyone else, but I prefer to attribute the failure of Chronos to flirt unmercilessly with me to the time consuming duties of being a mod.
Hey, don’t be raining on my parade, okay? Leave me to my pathetic fantasies.
Scotti