The autodiscovery of tracksback is a huge improvement for blog usability as most people don't have a clue what a trackback is - nor should they need technical knowledge. Autodiscovery (theoretically) handles a good deal of the work in the background allowing them to focus on the content.

However, some blogging software suites do not create the necessary RFD tags for autodiscovery to work. The blogger must provide the link manually in these cases. This is not possible in DNN blog without disabling autodiscovery and loosing the its benefits.

Could the Trackback URL field be present regardless of autodiscovery activation?

I recommend a notice above the field such as: "Trackback autodiscovery is activated for this blog. You may not need to provide Trackback URLs if the referenced website contains appropriate autodiscovery code. However, you may manually provide a URL here if this process is not successful." 

This notice would also help people understand the process a bit more.