As an example of how useful this is, I've created a virtual tag as follows:
$IsNull(<Display Artist>,,A)$IsNull(<Artists: Performer>,,P)$IsNull(<Artists: Guest>,,G)$IsNull(<Artists: Remixer>,,R)$IsNull(<Musicians>,,M)
With that tag in the main panel, I can instantly see which tracks I've already set the correct values for (and need to be careful about editing!), as well as which ones might need attention.