I think I'd personally distill that to "Not everyone can be the protagonist/antagonist of the story". Things fall flat if that's the case, as just about all of my favorite stories involve more than just a hero and a villain. There have to be others to make them shine - supporting members who guide the hero on his journey and minions who serve the villain, if not faithfully, then for their own agenda. "Supporting character" doesn't have to mean "bland" or "subservient" and I in fact have a ton of favorite characters in stories that in themselves don't bring about the dramatic conflicts but help shape the story, world and characters around them. They're living, breathing entities, and while they wouldn't make grand standalone stories, they bring something important to the table nonetheless. Where would Luke Skywalker have been without the sage advice of Yoda to guide him? How would Frodo have ever succeeded in destroying the One Ring without Sam at his side? Would Batman have maintained his sanity without the constant support provided by his faithful butler Alfred?
I know this role doesn't exactly radiate excellence and is far easier than stepping into the shoes of the grand schemer or the heroic saviour, but it's one that shouldn't be downplayed or overlooked.
I'd say the "essence of roleplay" for myself is being able to interact with a world and feel like it has life to it, depth to it. When I care more about what that Barbarian in the corner is thinking moreso than how powerful his axe might be, it's a success. When I know characters well enough to expect their surface reactions (and still be surprised the odd times they break that mould IC), it's a success. You need your Vaders, you need your Gandalfs, you need your Captain Kirks and Kahns, but it's going to be a short, uninspired ride without a dash of Lando, Gimli, or Scotty.
Again, I'm not here to say that being a developed character and being a protagonist/antagonist are mutually exclusive. In fact, if anything, I'd say those special positions are of the greatest import. Just saying that it should be understood that there's a critical mass for that sort of thing and the spotlight sure can't be large enough for everyone all at once.