It is a tough call. I would tend to agree with martianone on this.

With a 9-speed cassette you certainly might get away with a single chainring - even in very hilly or mountainous areas... that is, IF the bike were set up just right.

MTB (and touring) triple crank sets were initially set up for 5 or 6 speed freewheels. With fewer cogs, you actually might use a higher percentage of the cogs available to you within that range. With wider freewheels and later cassettes, there was just more and more unused overlap with each chainring effectively still really using only 3-4 distinctly different gears even though more incremental cogs were added. This means you could indeed really get away with the same number of different gears with perhaps a single or certainly a double crankset as you might with the traditional triple crankset.

The main difference (and problem) for more "modern" bikes when using double chainrings with a very wide range between chainrings would be that at some point you would be confronted with the necessity to shift down (or up) past several cogs to reach the desired gear change when you eventually change chainrings. This means with STI index shifting this is more difficult than when using friction-only shifting (basically you'll do a lot of repeated click-click-click-click-click before you achieve the intended gear when you shift between chainrings).

In my fairly hilly region I like using drastically different chainrings (something like 28-50) and 9-speed "road" cassettes of 12-27. However, using my friction shifters I can just slam-down or yank-up the rear shifter and run past all the intermediate cogs. So for me this is no problem.

As for touring range necessities, here is an actual example - from my recently purchased mid-1980s touring bike.

The original crankset on the bike was a triple with 28-44-50 chainrings and 14 17 20 24 26 for the 5-speed freewheel cogs.

With this gear selection, from small to large chainrings, you would use only:
28t = bottom 3 cogs
44t = bottom 3 cogs
50t = top 3 cogs

All the other combinations would be near identical and just repetitive overlap. Gears with this set up would range from a low of 29 to a high of 96 gear inches. You would NOT have the "nominal" 15 gears implied by the 5 cogs + 3 rings ... only 9 truly different gears.

With a 11-34 tooth 9-speed cassette and a single 38t chainring you would have a gear range of 30 to 93 gear inches... which is pretty darned close... and with no excess gear overlap. Yes, the chainline would be stretched to the maximum across the cogs at the extreme high and low gears. But, modern chains really are designed for a lot of lateral bending... and, depending on your terrain, you might spend most of your riding in the middle cog range anyway.

Remember, even with a single chainring, you might still need a long cage (MTB) triple derailleur to accommodate the broad range of cogs -since derailleurs actually do have a vague maximum cog size regardless of their total chainwrap capacity.

This could be an interesting decision for you to make. Personally, I tend to forget to shift when I should and wind up using far fewer gears than I have available. But for loaded touring this would be a pretty serious commitment.