I prefer unsupported touring for the flexibility it gives and apply the savings to B&B's, low cost hotels, and so on, in order to travel light!
I do one major trip every year (setting off for Spain-Portugal on Sept 4 for 6 weeks) and find it difficult to find riding partners beforehand. I suppose that's because folks are intimidated by long rides, don't have the time or the money, fear incompatibilities, and so on...
I do join up occasionnally with people on the road but I guess the major advantage of supported travel is having constant company and greater security. But then again, i would think this form of travelling is less of an adventure! And one travels with compatriots, with less opportunities to mingle with locals and discover new cultures...Having said this, travelling with one or no more than 3 other riders would certainly be interesting...
So here's my contribution to the debate! If you happen to agree with my approach, drop me a line. Maybe we'll ride together next year...
Louis T