@Applelight: Well, there's a difference in my mind between a supporting
character and a supporting
role. Supporting roles are characters like Cheerilee and Mac - ponies who make appearances every now and then, but largely are just around, making the world feel contiguous and familiar. Fluttershy's a supporting main character, though; she's a main character, but her personality works best when she has others to play off of.
Let's contrast Rarity with Fluttershy. Rarity can and has held up an episode practically by herself - "Sweet And Elite" only involved the other girls at the end, and aside from character moments like Twilight's adorkableness, the function the other girls served at the climax was as 'Rarity's real friends', not 'Twilight', 'Applejack', etc. (See the difference?) She's outgoing and hammy enough to drag an episode along by sheer force of personality (and is best pony for it).
Fluttershy, on the other hand, really gets to shine when she's in a supporting main role. "Dragonshy" is the quintessential Fluttershy episode, and it's one of my favorites precisely because of how Fluttershy contrasts with her friends. She's not the driving force of the episode, but it
would not work without her, because her personality and character are essential to it. Same with "Green Isn't Your Color" - her desire to make Rarity happy is matched by Rarity's desire to make her happy. So, yes, while she hasn't had a solo episode just yet, her character is still as rich and detailed as the other girls'. I get the feeling we'll see if she has the chops to pull off a solo episode in a week or so, too.