I understand and accept that. There are formatting things you can do in Evernote that can't be captured in Markdown, so those things would get lost when going back to Markdown.
I think I would be pretty happy even without true round-trip editing. I just find the Markdown style of input to be efficient. So I have been delighted to discover some Markdown shortcuts that already work in the Evernote editor (7.2 on Mac):
Typing "- <space>" or "* <space>" starts a bullet list
Typing "1. <space>" starts a numbered list
Typing "```<return>" creates a code block
Typing "- - - <space>" (three dashes followed by a space) creates a horizontal rule
If a few more Markdown-y things were added, I think I would be satisfied:
"#", "##", ... header styling
Syntax coloring in code blocks
Inline code formatting with ` ... ` (text between two backquotes)
Don't convert "- -" (two consecutive dashes) to a single en-dash in a code block
"> " to start a block quote
Paste Markdown text into Evernote and have it be formatted following the Markdown rules