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.
Here is what I do not understand: a student in high school can only participate a very limited number of times verbally in class because there are other students. In D&D there is a limited amount of magic items that need to be tried out. How does any DM calculate the involvement in such things into XP values given out after every session. My guess is that he/she would only be able to do this in very broad strokes, which does not seem to serve the purpose of XP as a measure of involvement of a player in the game.
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.
In a game like 5e, the death of a PC can be caused by one stupid roll of the die, especially in earlier levels. I do not understand why a player to whom the survival of a precious PC is paramount should be penalized by recieving less XP as somebody with a more reckless behaviour. I have played both types of PCs (but usually a more proactive type of PC) and both types of PCs are completely within the broad structure and genre of any RPG.
Thinking about what you have posted about the way you handle this in your gaming group, I remembered a situation in a Warhammer FRPG group that I played in around 1990. One player played a Wizard and she did not play aggressively because she was scared her PC might die. Now, WFRP 1st edition is a brutal deadly game in which a series of nasty d6-rolls will kill any PC, no matter how experienced. So her angst was valid. But also, WFRPG is a system in which individual XP could be given according to how well one roleplays the PC (which she was not the best in either. Not really bad, but not good). Others in the group did great, though and therefore recieved more XP. She eventually left because of this and we lost her as a player. I wondered back then if that was really neccessary and I still think that, no, it was not.
The issue of how you play your PC is really not an issue that should be handled with XP. Instead, it something outside of game mechanics and should be discussed to form a common basis on what the behaviour in the group as a whole should be.
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?
I do not recall the rule that dead PCs do not get full XP or less treasure. I am assuming they still get all the XP because they participated in the fight. And treasure for the same reason. And the fact that revival spells must be bought from the dead PC's pocket is certainly a valid way to handle this. But it is not the only way.
Why not find a way to handle this in which the player does not get the shorter end of the stick? Especially because in 5e death occurs because of a bad die roll in early levels.