You're really looking at two different scenarios here.
There isn't much difference between 1x9 and 1x10.
Biggest and smallest tooth count remains the same.
Only one more gear to choose from in your preferred range.
Price increase is bigger than performance increase.
Speed-of-wear increase more than speed-of-travel.
Requirements on adjustment accuracy increase more than any deliveries in performance accuracy.
But 11-speed has a considerably bigger big, and can even be had with a 10T smallest.
IMO, 1x9/10 drivetrain are for those who:
- can manage their rides in a kinda narrow speed span. Either through terrain choices or through personal fitness.
- have a morbid fear of front shifting
But yeah, a front shift is more critical than a rear shift.
If you're battling it out for parts of seconds and/or a bike length or two, then avoiding a front shift might indeed mean something.
For me, it doesn't mean squat.
Rider fitness, rider preference and ride characteristics has a lot to do with it too.
On my commuter I run a 3x7, although the granny gear pretty much only sees use in winters.
If I went for a bigger range cassette, I could probably commute quite comfortably on a 1x9. At least outside winter season.
Or an 1x10, if someone else would be willing to pick up the tab for the pile of chains, cassettes and chainrings my almost 7000 miles/year habit would generate.
But this is a known route, no long climbs.
When the cassette comes up short, it's no problem either coasting or honking it out.
As I said, a kinda narrow speed span.
I can certainly see it working out well for a certain level of technical singletrack riding.
A suitably snaky course limits the top speed.
And the really low gears are more suited for grinding up long climbs rather than negotiating a tricky section.
If you're one of those who go by car to your riding spot, no need for a good "travel gear" either.
My XC marathon "race bike" runs 3x9.
I don't really use the granny there either. But that's the drive train it came with. And it's OK. Parts are cheap and last fairly well.
On a 60-mile race course, there can be some sections of blacktop or nice gravel sections, so a decent high gear can come in handy.
And I'm neither the strongest or youngest rider out there, so I don't mind dragging a bailout gear around either.
The 11-speed is/can be a different beast.
With double/triple cranks, there's always an overlap.
Crude rule-of-thumb says you have about 2/3 as many usefully different ratios as you have combinations.
So a 2x10 might yield about 15 usefully different gears.
11-speed and a 10-42 tooth count will give you 11 different gears, no repeat ratios.
Might be worth giving up those few gears in order to get away from that pesky front shifting - if you have the cash.
