Just thinking, but I don't think Ridley's importance is a real factor when it comes to the possibility of him being boss-only. If a boss-only character is to have little relation to their correlating game, then why would the strength of their role matter?
Just a thought....which leads to another one.
I never played Kid Icarus, but how big would one consider Palutena's role to be? She's a goddess who basically gives Pit his abilities, I believe. Maaaaybe her role can be compared to...Zelda? She's not a damsel in distress like Peach, and while Zelda has also played the "rescue me" role, we all know Zelda does have some power of her own. But in the end, Zelda is playable, and currently Palutena is merely a weapon in Pit's arsenal (a powerful one, but a weapon nonetheless).
So...can we really say role matters? Is it a viable argument? I would think Ridley's strength of role is greater than Palutena, and definitely Petey....but if boss-only characters are treated like I believe they will be treated, will role strength matter?