And that's not because they necessarily think that heavier bikes are better for their 'fitness' it's because they are both (Ferrand-Prévot and Vos) superb XC and MTB champions and have enjoyed success in those disciplines since youth.
No, not because they're heavier bikes (and I never said it was) but it is good for fitness because even the mighty Vos thinks she'd struggle to keep motivated plugging away with the same bike all year long: "each has different variations of training, different races and different people, so I don’t get bored by doing the same thing. Mixing my disciplines helps me to stay focused the whole year and they help me to become a more complete rider."
in a road.cc interview
I guess you will get some people who enjoy a nice heavy bike and work harder as a result
Except in high winds, you don't enjoy the weight as such, but more that you're not sat on a razor blade exposed to the water, mud and oil due to no mudguards or chainguard and that heavy bikes can have all mod cons like better navigation aids, food and drink set out in front of you, a sound system, hot coffee, heated gloves and overshoes, perpetual lights, a sofa... (and yes, I've seen all those and more... although the sofacycle is getting up there with the pub bike as specialist kit, really).