Oh dear, I knew you'd run into trouble as soon as you tried to make the numbers work for you
(oh I do so love it when other people justify my own misconceptions )

My much maligned commuter, the brute that spawned this thread, wears an mtb crankset. The rear cassette is actually the same as my Jamis (btw, I really like SRAM cassettes, but that's beside the point).

That bike gets ridden mainly on the flats. Adelaide is a large, flat city surrounded by hills. I live in the hills but when commuting, park at the bottom of the hills and ride in from there. Yes, I know it's lazy but it's still a 10km commute and to go all the way home requires negotiating a very dangerous bit of multil lane road with a 70km/hr speed limit, then a sharp climb that hits 10% for a sizeable slab of it and which usually takes me a good 20mins followed by 6kms of rolling 80km/hr road - at the end of the day, I'd rather drive that.

So I choose the lazy option and commute on the flats. My commuter is what you'd call 'weighty' - she's a beefy touring frame with one of the heavier Brooks saddles, carrier, mudguards, thumping great big steel hitch for my wee daughter's tag-along, hefty U-lock and typically, over 10kg of text books in the panniers. Not heavy weight touring but lower gears are preferred to higher gears.

That's to set the scene.

She's got an mtb crankset ... because when I built her up, I was able to get a second hand crankset for not a lot of money (and the sodding thing has cost me heaps in other bits trying to work around it ).

The middle ring on the commuter is ... wait for it ... a 34 tooth!
Just like Tim suggests.
And I hate the useless thing.

You get the bike rolling, pop up a gear or two and bing, you need to be on the big ring. My big ring is nothing like the 50 you suggest, possibly a 42 but I don't know at this time.

To make any use of a 34 on the back, I'd need to be able to run it right across onto the small cogs ... which I can't do with an mtb front dr dammit though I could with a road dr ... which then wouldn't shift with my mtb shifters (did I mention this crankset has spawned other issues?).

The main point being that 34 is ludicrous for flat or even undulating bike path running - she's often on a narrow bike path that follows the River Torrens and is up and down the bank like a dancer's knickers. The jump between the 34 and the big ring I carry now is enough to annoy me, increasing that jump even further just justifies my choice of a triple set up.

Tim, you are a far more forgiving man than I am ... but I'm old enough and grumpy enough to refuse to put up with too much bulldust from machinery.

Now, if I think about the undulating territory around home, such as my favourite 20km loop where I need and use the full range of gears on my middle and big rings but not the granny, having to muck about going from big to middle would drive me nuts, especially with the extra shifts needed at the back every time you change chain ring. Maybe there's a difference in riding style. I change gears a lot and work to stay in a relatively consistent cadence zone and when you lack torque like I do, you change often.

Richard
(at uni ... procrastinating)