If I live in a socio-economic system where I have to have money in order to survive (let alone do anything else), I'd probably pick the higher paying option so I can ensure I have enough to live and maybe secure some kind of savings in the long run for other things.
That choice is a mere byproduct of a system I live in and doesn't demonstrate anything besides the premise that a person goes for the higher paid option simply because it offers more access than a lower paid option so they can ensure they don't have to worry about those things in the first place.
But... financially situated people... or even those who have their needs met already would probably pick the low paid option (posting on TrekBBS) or even do it for free - myself included (plus I've already done that).
If you eliminate money from the equation though completely and base an economy around access abundance and user-ship, if there's a need to mop a floor of a Wal-Mart for an hour or two, fine, I'd do it (and with high enough rotation of people, you'd only need to do it for a fraction of the time, which would take say an hour depending on the size needed to be cleaned) - even though it would be unnecessary since that can also be automated.
Anyone who is willing to see can clearly see that money (or whatever rewards you'd like to substitute) offers an incentive to get people to do jobs that might otherwise go unfilled. It can help prevent shortages in some career areas and gluts in others. It can help insure that trained workers are available when needed to do the job. Your idea that someone competent will always be willing to step in when needed is just wishful thinking.