I'm really glad I asked about these settings, as I almost didn't. After saying I couldn't ever see myself using the [Custom] field, something came to mind that might actually make things simpler for me.
At the moment, if an album features various artists on individual tracks (but is not a compilation), I keep the [Artist] metadata the same as the [Album Artist] for all tracks and put the featured artist details into [Comments]. This is just the way I've found works best for me. For example:
[Artist] Raekwon
[Comment] Featuring: Ghost Face Killer, Master Killa, Method Man, The RZA
But if I create a custom [Featuring] field, I could have it like this:
[Artist] Raekwon
[Featuring] Ghost Face Killer, Master Killa, Method Man, The RZA
This would free up the [Comment] field for anything else that it may already populated with (recording venues, dates and other such things), and I think it also looks tidier. I'll give it a try and see how it works.