Yes, this is exactly my use case.
It gives a very direct feedback about your syntax: you don’t have to look to the right pane every other second to check if you aren’t making trivial syntax mistakes.
It would give a bit more structure for those who are less apt in Markdown (any beginner).
Of course you still have the rendered html on the right side of the composer window.
I would see no immediate disadvantages to this, especially if it’s an option.
PS: Apparently there’s not even a Markdown syntax highlighter installed for codeblocks 
:
# Markdown codeblock's syntax...
... unfortunately ...
- isn't
- highlighted
**at all**!
EDIT 2 hrs later: Fixed by @pmusaraj