(Warning : Super pedantic response on a completely trivial matter)

Ok, I've just tried it and I don't like it. I don't see how it's a shortcut when it still requires two inputs, the CTRL + i to initiate and then the necessary use of a cursor key to move over the trailing underscore. My way is just two inputs too - two asterisks (I use asterisks, you can also use underscores) that are typed in order, so no funny cursor movement.

And the point of Markdown is that it's intuitive - markdown is meant to look like 'marked up' text. Putting stars around a word clearly indicates some sort of emphasis. That's easy for someone from any culture to grasp. Italics is cursivo in Spanish and 斜体の in Japanese. If they don't know English (and why should they have to?) then it doesn't make any intuitive sense to them for why they're using (and having to remember) the first letter of a word they don't know in a key sequence to emphasise a word. Imagine explaining CTRL + i to a class of Swahili speaking kids, rather than just showing them how to directly put stars (or underscores) around a word for emphasis.