You meant TMDB, right?
It couldn't be relational, because several programs I watch move between Networks, when one drops them and another picks them up. Or do you mean the "Original Network" field on the TVDB?
No, I definitely mean TVDB. I use it for TV Shows, not Movies.
I did mean Original Network. They changed their tables at some point and renamed the original Network field to Original Network, and added the detail about multi-network shows to the Network field. That would be too challenging to pull in automatically, but Original Network should work just fine.
Personally, I don't care about distinguishing between series that move between networks and preserving the various networks the show has been on historically (which is why I just use a Relational field). I just like to be able to filter my series to only show HBO, or Hulu, or Amazon shows. And, perhaps more importantly, I have a bunch of clever Links set up which, when you have a show selected where [Network] is HBO, it'll take you to that show on HBO's streaming site, and when it is on Amazon, it'll take you to that page on Amazon, and so on and so forth.
I've been fine filling it in manually, since it only needs to be set once per series. But since the detail is in TVDB, and MC pulls lots of other stuff from TVDB, it would be much more convenient if MC did this step for me.