I see quite a few of these people on weekend mornings. I do wonder, if they're cycling for fitness, why don't they use heavier, less top-of-the-range aerodynamic bikes? By riding a bike which is requires more energy input to maintain speed, would you not get more out of your exercise for the same amount of time?
This also applies to lycras as well. Wearing normal t-shirt and shorts would be less aerodynamic, but it would surely force you to increase your energy output. I'm think of taking up cycling as an regular exercise, and I wonder whether I would be wiser to use my commuter bike which is on the heavier side at 14kg instead of selling it and investing in a proper sporty road bike.
Man, you got beaten up here didn't you?
I do, and started on a 16Kg Trax TFS.1 full suspension (if you can call it that) mountain bike and yes, Biggin Hill is a different story on a 9kg road bike
Cargo shorts are OK for a 30min blast around the woods, but you'll hurt yourself on a 3hr road ride. Appropriate bike wear for the ride is the mantra.
My buddy asked me the same question; surely Id get fitter heaving The Tank up and down the hill?
Well going uphill it is harder work, but you have to think about the whole ride, and the total rider+kit weight, and soon a few kilos here and there aren't such a big deal. Being heavier I come downhill a bit faster, but on the flat once you're rolling, it's really neither here nor there.
Then you have to understand exercise, particularly endurance fitness over strength and sprint speed. The short story is that you need to exercise in such a way that your heart is under a relatively low stress over a long period of time - usually referred to as Zone 2 training. Huffing and puffing red faced is Zone 5 and is of little use long term.
Now you could do either of these on any bike if you pick the right terrain and speed...so it should be clear to you now why he heavy bike doesn't make you fitter quicker.
The last part of the question is, why get the lighter bike? Well, the answer should be "Don't." BUT. If you feel like a nicer, better looking, better handling bike and/or with more durable components and reliable shifting and braking, that is set up for leisure rather than loaded down for commuting...then get one
I'm assured that there are some comfortable cargo shorts around. Appropriate clothing for the ride is the mantra... some bike wear is spectactularly unsuitable and may well be regarded in future much as we regard tweed rides now. Heh, I wonder when "Lycra Rides" will become a historical event 
