That's great, Palmera. And tends to confirm the old adage that "we become what we think about."
Will everybody in ERE always have more than one income source?
I don't know if I would go so far as "inevitable" but I don't think it is difficult to bring in a few extra hundred a month. For most of my adult life, I have always had some sort of side income that brings in around that much. Most of these side jobs have been doing things that I do genuinely enjoy (maybe not quite so much that I would do them for free, but enough that I would probably still be doing them at SOME level regardless of income potential), so I can't see that I would ditch them when I achieve FI. For the kinds of people who are going to aggressively pursue FI, I think it is likely that non-investment income will not drop to zero once FI is reached, because the kind of person who does that is also the kind of person who is unlikely to just sit home and veg out in front of the TV for a few hours.
I don't consider myself unintelligent or skill-less, but I have never had a side job fall into my lap that I remember. So I am doing my ERE planning assuming no extra income. But "when is enough" is probably more a question of temperament and how much you hate your current job. Making a few hundred a month should be doable for most people with a little hustling--clean a couple friends' houses, for instance (if they're not ERE!). But if you'd need more than that to be able to "retire," I think you better have a real plan and know your market.