You can play with them and keep the cards you acquire while playing, but you do not gain any rewards from scenarios or adventures your character has already completed. That includes even the "Each character draws a random card from the box" kind of rewards.
No character may gain a reward from a given scenario, adventure, or Adventure Path more than once.
So you might acquire some better cards, but you won't get any more skill, card, or power feats by replaying scenarios.
Since a player who has never completed the scenario can receive the reward for completing it, that is allowed by the rules.
As to the spirit of the game, if the "new" character who is able to earn the reward is going to be a part of your party and not just a loophole to get the loot again, then I'd so no, that doesn't violate the spirit of the game.
But if you are just grabbing a random character and dragging them through one scenario so they can get the loot just so you can than ditch them immediately after, then yeah, I think that does violate the spirit of the game.
Under "Things To Keep in Mind" the rulebook says this:
Choices Matter. Your choices have consequences. Once you choose cards for your character, you can’t trade them for other cards whenever you like...Every choice matters—take your choices seriously.
Using a "temporary" character to get a loot card you chose not to take before is trying to avoid the consequences of your choices.
I think getting the weapon is totally fine in that case. It wasn't that you couldn't use (Even with out proficiency, you can use all the weapons. You just have a penalty for doing so.)
In fact, technically, someone has to take the reward, at least for a moment, because the scenario gives it to you. You might decide not to put it into a deck, but you have it at least for a bit.
Assume for a minute that other character wanted it. And then after a scenario or two decided they didn't like it. Would you have any qualms about taking it then?
I think getting the weapon is totally fine in that case. It wasn't that you couldn't use (Even with out proficiency, you can use all the weapons. You just have a penalty for doing so.)
In fact, technically, someone has to take the reward, at least for a moment, because the scenario gives it to you. You might decide not to put it into a deck, but you have it at least for a bit.
Assume for a minute that other character wanted it. And then after a scenario or two decided they didn't like it. Would you have any qualms about taking it then?
I would take it.
My friends and I did away with that rule of not getting the reward for replaying scenarios. The game is still very fun and challenging, we even added a "final kill" rule where whoever lands the final blow to the Villain is the only one to get the reward. The game can be modified in so many ways and can still be awesome to play. It has not lost any value to my gaming group by changing the rules.