I see two situations where the simple Sram two paddle system is not optimal:
Starting or finishing a climb
My Di2 is set for shifting 3 cogs with a "long press" (over 1/2 second).
In the big ring, arriving at the base of a climb, I
long press both bottom levers for 1/2 second. That shifts to the 34 from the 50 chainring, and 3 harder cogs in the back. That's a good start to the climb. I might shift one more cog right away, depending on the grade of the climb.
Starting on the downhill, it's a
long press of both top levers. That's the 50 chainring and 3 easier cogs in the back.
Not noticing which chainring is in use
Occasionally I think I must be in the small chainring, and hit the top button to shift to the big ring. But I'm already there, so nothing happens. Holding both paddles on the Sram shifters would drop it back to the 34.
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I think the
"double shift" in a previous post is referring to something like the old
"half-step" gearing. The 2 front chainrings and 5 cogs in the back were set so that the small chainring shifted to the next gear ratio in between the two adjacent cogs with the big ring.
Something like this chart, half step 42 and 46 chainrings, with a 24 granny. The two larger chainrings have to be close in size for this to work, not like a 50/34. And the few rear cogs are widely spaced, not like today's 10 or 11 speed cassettes.
The rear cog shift on a Di2 is instantaneous. I was clicking one more cog every two or three pedal strokes, under load, when the hill kept getting steeper today.
Shifting the front isn't that fast. I wouldn't like automatic computer controlled front chainring shifting.