Silliest thing ? Not really, I happen to have a 1986 Fuji manual right in front of me, The Mt Fuji mtb has a 28/45/50T crank gear and a 14/16/18/21/24/28 rear cassette. How many SS/FG's come with a 50/14T setup ? Even with a wheel size difference that 26 x 2.** vs 700C x **, that mountain bike has more GI. The late 90's Fuji MX-200 I have, 48/14T if I counted it right is the top gear and that calculates to 89.1 GI. From what I've seen on line, you might be able to find a 48/15T for a SS/FG with 700C's, that's 84.5 GI ? Close but still less. Maybe newer MTB's have different gearing/speeds that more closely approximate what SS/FG's have, but the mtb's all pretty much weigh appreciably more than a SS/FG and some by a considerable amount too that will get the mtb to cruiser weight class. Then you factor in the rolling resistance of the different width tires and an mtb allows a cyclist a harder workout to be selected. Granted you can make any bike have a harder GI load, but my statement was a generalization for what many either already have or can get as a standard off the showroom floor purchase, not customizing anything. And if you throw a 12T on the rear of a SS/FG, then customize the mtb just the same to be fair about it ?

A little leg weary after a few miles of a ride, 5 GI into a relatively strong head wind can be enough of a difference, that a cyclist would switch back and forth between gears a few times that are about 5 GI apart to find the optimal cadence and speed. Sometimes, even the geared bikes can leave you in between the optimal selection.