We have recently found that we can spend a lot of time editing text versions, especially if they include rich text tokens as these don't appear properly. Have to manually input the text for those tokens.
Is it possible to find out in Marketo how many of your contacts actually receive or open text version?
Everyone will receive both the HTML and Text version of an email; the version that displays in the inbox is dependent on the email client. Since the Text version has no HTML, including the open tracking pixel, I'm not sure how you could track it other than appending something to the end of the text version's URLs to track clicks...
Yes, this is something I've done in the past to include something on the end of text links. Otherwise there's no way to tell which version a person viewed and acted on.