If I read this right, essentially you're wondering why any player should bother writing a role, if they can get the special kind of roleplaying rewards without one, and at the same time stay flexible in adapting their character as they want to.

The first thing I'd say is that while a role isn't required to get rewards, the advantage that a character with a detailed role has, is that we notice them much more quickly. A good role usually says "Hey, I'm interested in in-depth character development, pay attention to me!". While this doesn't guarantee we'll do so, or that we'll like what we see when we do watch you, it makes it much more likely.

Secondly, a character with a well developed role lets us get a feel for what the character is (or ought to be) before we watch you, so you're already a bit further along than you would be otherwise in us being able to assess your roleplaying ability. If your role sucks of course, or you're not following your role, that's bad, but no worse than if we see crappy/inconsistent roleplay from a roleless character. To put it another way, if I see someone being inconsistent (in my view), it looks bad, role or not. I'm going to watch a roleless person a lot longer, on average, before I can decide if they're consistent enough to qualify for any kind of special attention/treatment, just because I don't have that unchangeable role of theirs to hold them to as a standard.

This gets to my final point - If you're playing a character with nonstandard attitudes/actions/tendencies/beliefs/whatever, a role is very much in your interest. For that Scion Bard who wants to betray them to Battle, as you used as an example, having a role from an early age means we're aware of your intentions, rather than tending to go with the assumption that you just got bored of scion/tired of getting rager-ganked/etc. Getting back to consistency, if your character *does* have a mid life change of heart, we're a lot more willing to see it if you've been roleplayed consistently throughout, and have developed the character such that we can follow and track that, and maybe we understand that your elf rager has finally decided that he just can't bring himself to kill good aligned mages anymore because that struggle has been a lifelong theme for him, rather than thinking you're just a tool who was tired of getting killed at a time the cabal was weak.

So no, you don't need a role. But it's definitely to your advantage to have one, even late in life.