I think giving all the secondary characters a bit more prominence couldn’t hurt, really. The Cakes are Pinkie’s living-mates, Spike is kind of a major character, the Royal Couple are definitely important, etc. That includes Celestia and Luna, as they are both pretty major, and very important. I just think that Celestia’s short appearances make sense for her, since she is Twilight’s mentor. She’s going to be listening as the ponies report, or something along those lines. Luna is nocturnal, which made her an obvious choice for the Halloween episode, since it’s a nighttime holiday. It was really just how the mechanics of Season 2 worked out, in my opinion.
Though I would like to see Celestia have an episode, of course, I’m not sure how likely it would be, seeing as how her role is pretty much just a character to show lesson or listen to what the ponies learned. Like we said earlier, though, a flashback episode is possible, and that would give screentime for both the sisters and open up more time for Discord, which I’m sure a lot of fans (myself included) would enjoy.
However, I don’t think we can really complain about Luna’s screentime in Season 2, as she was immensely prominent, compared to others, especially in dialogue and characterization. It’s fine and dandy to hope for more screentime, and I’d approve of that because it’s natural, but I don’t think any of the secondary characters were fucked over by the show in Season 2.