Plex requires tv show episodes to have file names in a very specific format. This requirement results in an incompatibility with tv shows managed in an iTunes library. Strangely, Plex accepts some iTunes tv shows while rejecting others, even though they may follow the same naming conventions.
There is no reason why Plex should rely solely on tv show filenames while ignoring the ample metadata available within each file. Plex works just fine with managed iTunes libraries when importing movies, so it should work equally well with tv shows.
Given the large number of users who store their media in iTunes, it is in Plex's interest to provide a solution for this issue rather than expect their customers to choose between Plex and iTunes. At minimum, they should develop an agent specifically for importing tv show content from a managed iTunes library.
Edit: moved to the Mac section and rephrased as a feature request rather than a bug report.