I find this very interesting and have a couple of questions regarding this thing:
1. What constitutes "taking risks"? Taking part in combat? Being the first to climb a huge cliff? Does not everybody in your group participate in combat, the most risky operation of the game? And is the fighter in the front not more in risk of death than the archer in the back? How do you calculate - XP wise, mind you, in specific XP numbers - that risk?
Taking risks can be anything from being the first through a door to standing in against a powerful foe to trying out an unknown magic item to all sorts of other things.
And while most characters try to get involved in most combats I can't count the number of times I've seen players pull their character(s) out of the fray at or above half h.p., leaving other also-hurting characters hung out to dry.
The archer in the back, if archer is all she is, does what she can and risks getting clobbered with area-effect spells just like any other back-liner. But if said archer is being an archer just to be safe when she could instead be just as useful (if not more so) up in the front line, that's annoying.
As for xp, we don't calculate it down as finely as you are looking for here. If a character gets involved in a combat it gets xp for it (unless it dies, in which case it gets half); a character who does nothing gets nothing.
2. And what if the Passenger Character (not the player) simply lacks the mechanical skills to accomplish what your more active PC is doing? What if the player plays a highly specialized Passenger Character who can only shine 1 out of 3 times?
That's a different thing entirely; if a character is a passenger for two encounters because it can't usefully do anything other than toss daggers, but then it blows away the third encounter almost singlehandedly, I'm cool with that; it's doing its bit where it can. (Illusionists in an undead-heavy dungeon are my usual example here) It's the characters who *could* do more on a regular basis but don't that annoy me to no end.
3. How do you regard player personality? There are more active players who are very active in the game (I am one of those). And there are more passive players who play for different reasons. Do you not want to play with them, even if there are your friends? Will they not leave if they get less XP? What if the game does not offer the chances for everybody to contribute equally? Everybody cannot shine all the time. I feel that since this is a group effort, everybody has to let everybody else shine, too. What if your (or my) style of play is more compatible with the style the DM is used to? Can this not lead to a situation in which someone in the group gets branded playing a Passenger Character but in reality is simply not challenged in a way that is more appealing to him/her?
All valid questions and points, but none of them excuse the player to whom their character's survival (too often at the expense of other characters' deaths or other sacrifices) is paramount. And if they're going to leave because they get less xp that's maybe not a bad thing, even if it's someone who is otherwise a friend.
And to haul this back toward the original topic, the PC of a player who isn't present is still a PC in the party and is still expected to pull his weight (based on established character). If I've got a character who tends to wade right into combat when I'm playing her I sort of expect that's how she'll be played when I'm not there; and if she dies so be it.
4. Is there a discussion at your gaming table about what kind of involvement the group wants to have regarding taking risks (for example: "You are heroes who take risks!", "It seems to me that you, Argon, were holding back in the Orcmine. We really needed your help with that Shaman Goblin Queen but you just waited - we are in this together, so man up!")?
In different words I've had almost this exact discussion with other players in a game, getting a response largely consisting of (paraphrased) "I'm not going in where I'm probably going to die! I'll stay back here, thanks." The logical conclusion, of course, is if everyone starts thinking like this we might as well call it a day and go back to town.
So to use your example above, when the dust settles after the Goblin Queen battle Perrina (a front-liner) is lying dead on the floor in large part because Argon (another front-liner) didn't stand in and bail her out; Argon survives and (if group xp are used) gets full xp and a full treasure share while Perrina may or may not get full xp and treasure for that battle but gets nothing going forward and also has to pay for revival at some point - though if a Goblin Queen is a major foe she's probably at a level that can't afford revival spells yet, making her SOL. How in any way is this fair to Perrina and-or her player?
Lan-"apologies for any incoherence above but it's late and I'm falling asleep"-efan