Yes, you are correct.

In general, harder/slower bike = tougher workout for same given speed. Of course, if you just ratchet down the effort on the slow or fast bike, the workout will be inferior regardless of bike.

Roadies who are hammering hard NEED their fast bikes to keep up with fast guys they train with. It isn't a matter of just feeling the speed - if you brought your mtn bike to a roadie ride at your level, you would get dropped very quickly.

I have gone on a few intro-level cyclist as well as triathlete bike rides on holidays that seem to attract a lot of weaker/newer riders. I would have no fun if I brought my road bike and rode away from everyone on the first hill, so I bring my mountain bike to give myself at least some challenge.

The only problem with using these clunky slow bikes for long distance training is that you aren't training on what you're racing on. If you need to ride for 2-3 hours or more, you'd be best on your steed of choice, as the position can be quite different between bikes, meaning that even if you're in great mtn bike shape, you may have some back issues on the roadie at that length of time.