I'm sure this has been asked thousands of times, and previously ignored because there was no possibility of checking automatically. However, now that the parsoid / linter extension combo is able to detect and report markup errors this might be a perfect time to create an automatic log / graph of errors introduced by each of these tools to make it possible to somewhat measure if the visualeditor really lives up to its aspiration of reducing mistakes.
This may also help find previously unknown errors caused by either of these tools, so having such logging will certainly be a good idea anyway. This data could also be mined by automated tools such as bots and ORES to facilitate its cleanup.
Considering that smaller wikis don't have bots to regularly clean up these errors, it might also be useful to evaluate how long these errors tend to stick on articles, especially for errors that prevent the page from being viewed properly.