Someone on a 25lb bike, riding the same course and same hills, will have to work harder and get more exercise, and will get stronger than someone on a 10-15lb bike. (Assuming, of course, times are not too different, and overtraining is not happening.)

Someone on a 45lb ride will achieve even more. The added weight could be in panniers, so the bike itself doesn't have to be so heavy. And the weight could be very adjustable. Weights from a weight machine or barbells would make it was to keep track.

A racer who actually went on a long tour with a very heavy load commented on noticeable strength and fitness gains from having to deal with such a heavy bike.

Heavyhands work on a similar principle, and there are marathon runners who sometimes train by wearing extra heavy footwear....

(Obviously those who are actually in a race wouldn't do this on race day, and the same would apply to those who have to keep up or compete with lighter competitive bikes in group rides, unless they are strong enough to make up a reasonable weight difference, or up their output. So this is meant to apply mainly to recreational fitness and training rides.)

There are many other reasons people like or gravitate towards lighter weight. Just from a purely fitness/exercise perspective, though, I'm starting to feel more accepting of heavier bikes.

Part of me thinks a sub14 or sub-10lb bike would be awesome. But another part sees it as very unnecessary and questionable, and would rather have the extra thousands available and at my disposal than not.