I'm not disputing you -- I don't know enough yet to have formed an opinion -- but help me understand what this means: "Characters who died before the encounter took place, or did not participate for some other reason earn nothing." That's page 37 of the DMG. I guess I'm defining "participate" wrong? I have indeed been omitting awards to a certain character who just sorta stands back and does nothing when combat breaks out. But for your interpretation, "not participating" would require being absent from the dungeon or encounter area entirely, right?
Well, by a very very strict literal interpretation, you are correct. However, it's not very practical, and sooner or later a player will feel someone else wrongly got more XP than he did and put you into an uncomfortable position of justifying your decision. It might result in hard feelings -- he might start thinking you have a bias against him even if you're playing it fair -- and that's bad for any gaming group.
The more practical interpretation is this: a player earns his full share of XP for an encounter unless the character is incapacitated before the start of it (ie, they are dead, dying, unconscious, paralyzed, petrified, etc.)
If you fight a Medusa, she wins initiative and turns the Ranger to Stone ... the Ranger would earn XP from the Medusa because he started the encounter fine. He was petrified after the encounter started.
Once the Medusa falls, the party starts to drag their statue friend to town, and a Mind Flayer comes down the tunnel. The Ranger will not earn XP from the Mindflayer because he was incapacitated before the fight started.
Now, the Ranger might still complain about getting less XP, but you can objectively prove that his character did nothing because there was absolutely nothing he could do (and everyone at the table was a witness.)
You could argue that a player who has his character stand back and do nothing for the sake of being a dead weight shouldn't earn XP, but I'm not sure I'd invite that guy back to my table at all.