I personally think that this discussion is turning into my triple is better than your double and double is terrible. Whereas the double camp says otherwise.

First of all, I have toured with a triple. It's funny that nobody recognized the Sugino Super Maxy triple crankset. It's an OLD SCHOOL triple crank known for its good quality in the 80s so you know how long I've been touring. It's 110/74BCD which means, it is a triple not a double. Not sure what so many people here think that with a 110/74BCD, you can not choose a lower gear. Mounting a 24T on the 74BCD is as low as you can get it on. Anything lower than a 24T needs a mountain triple crank!

There seemed to be a confusion that all double cranks are all a like. This is completely false. A road double crank is a 130BCD and the smallest ring is only a 39 or 38T. A compact crank is a 110BCD and the smallest ring you can fit on is a 34 o 33T. A mountain double crankset that can be adapted to road touring comes either a 45T/28T or 40T/28T.

The double crank we are taking about in the double camp is NOT a standard crankset unless you buy a Rene Herse double crank from Compass cycles (Jan Heine's company) or you make your own with a 110/74BCD triple crankset turned double. We call this a touring double or mountain double but this confuses with the SRAM double that comes standard on the Salsa Fargo.
With a touring double, 42T/24T or 40T/24T is possible. Compared this to a stock 46T/36T/24T or 26T Sugino triple crankset (XD-600 or 500 or equivalent) and the 42T/24T, you have exactly the same low gears as a triple, because the triple has the same chain ring in 74BCD as the touring double. I don't understand why there are so many people here still think a touring double with a 24T chain ring can not give the same ratios as a triple with the same 24T as its smallest chain ring. Math I suppose must not be their most strongest subject in high school. There is no difference in low gearing performance between a triple and a touring double.

The difference is in the middle and higher gearing. Let's take a 46T/36T/24T for reference purpose against a 42T/24T touring double. Against a 46T on the triple, the 42T on the double is about 10% lower in all gear ranges. This means, you are most likely to be using about 1 gear higher on the 42T to make an equivalent 46T with the rear cassette setup. The difference is not all that noticeable. I know as I have both systems. Now, a 42T against a 36T on a triple however is slightly noticeable because it makes a 42T 16% higher in all ranges against a 36T middle ring setup on a triple. This means, you are more likely to double shift to your 24T with a 42T whereas you may stay on a 36T with just 1 or 2 gears left. That's the only downside, but this does not mean you don't have all the required gears because once you are on a 24T, you will have exactly the same ratios as you would with a triple in the lower gear ranges.

There is absolutely NO problems climbing a 18-22% grades with a 19" gear unless you need a 16". But then, you will need a 20T small ring but that's available on a mountain triple crankset as the 74BCD can not take anything smaller than 24T.

A touring double is not recommended if

1, You need a 48T to 53T chain ring (apparently some people think they need and can go fast with a 48T or 53T or 120" + with 4 panniers)
2, You need a 16" gear or lower where a mountain triple crankset can accommodate

But these are the 2 extreme cases. It's like telling someone you need 4 spare tires on a car because it's likely all 4 tires can go flat. That's true, but how often that's going to happen?

We can speculate on extreme cases, but I think the majority of cases people are just going to use what ratios they want. Triple is a no brainer because it got all the ratios, but a double is sometimes a necessity for people like myself who has a double shifter and is a huge expense going to a triple because I have to change shifter, front DR when I can just change to a triple crankset 110/74BCD which in my shop happens to be bountiful (people upgrading to fancier inboard cranksets) so it lowers the cost. With people doing this and seeing them last year on tours goes to show that a touring double is feasible. But sometimes, it's just not everyone's cup of tea.