Perhaps cyclists new to the idea of touring think "Tour de France"?!?
As I said before, I think a consistent average of 100-150 a day wouldn't be too fun. However, I do like to find opportunities on tour to cover long distances when its convenient, downhill, or when we can assure ourselves some rest days.
My wife and I will often plan to do a long ride near the end of a tour, or before a break. For example, last summer we were bisecting Japan, riding back from Sadogashima to Numazu. We'd taken it slow getting there, going about 50-60 miles a day through the mountains of Yamanashi and Nagano prefecture. On the way back, we decided that after we'd reached the crest in the middle of the country, an area called Nobeyama pass, that we'd just continue to ride overnight and try to make it home in one push. We ended up riding about 200 miles in a 20 hour push from Nagano city back to Numazu. It was one of the best experiences I've had on a tour, partly due to the fact that we we alone on the quiet, country roads in the middle of the night, but mostly because we were sharing an adventurous change of pace with a loved one.
That said, I think that long days of riding do have a place on a tour, as long as you can justify the extra expending of energy with a day or three off afterward. We've used the same tactics to cover long distances of trackless wilderness in the states too. Also, sometimes you've got friends to stay with in point A and point B, and its worth busting ass for a day, or overnight, to get there and relax!