I don't think many people will use it efficiently, and there will be more than likely loads of abandoned questions and answers sat in storage doing nothing.
Then either storage is full up of pointless data, or we need some policy to clean up, such as inactive are deleted after X days, then we need some script to monitor and check if inactive, and something to store and check when last activity was, and so on.
For the very small potential usefulness this may bring to a small number of users, I just don't see it being worthwhile at all.
You don't need to anyway:
You can use the markdown characters in a text document on your PC by wrapping the text in whatever markdown you want, which your editor shouldn't translate (basic editors won't).
For example, in your text editor type:
*I want italic*
**I want bold**
When you copy paste the above into Ask a Question or an Answer text area on a Stack Site, the above will be automatically converted and look like:
I want italic
I want bold
A header is just hyphens under a word, eg:
Some Header
----------
When pasted into Stack, it would again be automatically transformed into markdown and would look like:
Some Header
Everything is the same, using bullet points is either:
Unordered list = 1 space, a hyphen, 1 space then your list text
Ordered list = 1 space, a number, a full stop (period), 1 space then your list text
Then proof read it to make sure it's formatted correctly, which you would do writing it on Stack anyway, if it's going to be a "very complex" question.