People being frustrated because others get something they get, too?
At the risk of delving into politics, this is the crux of the difference between socialism and capitalism.
In the realm of this discussion, it is "get" versus "earn". It is human nature to want to be rewarded for one's effort. When the reward is unequal for equal effort, or when the reward is the same for different levels of effort, it takes away the incentive for making the effort. If all you have to do is show up, players may not take greater risks when there is no greater reward.
Let's take an example.
Player A works his butt off in the game. Researching clues, thinking of clever tactics, working out the logistics of hauling rations and loot, and being bold and adventurous.
And then there is Player B. He spends most of the night eating Cheetos and the group's snacks. He will roll the dice when it is his turn to make an attack in the combat round. But that is about all he is good for.
So in Scenario 1, we use the earned xp method. Player A gets more xp than Player B because he made the effort and did more. Player A is pleased because his rewards matched his effort. Players C through F see that Player A earned more xp, and they endeavor to increase their efforts in the next session. Player B may or may not care that he earned less xp. But if he wants to earn more, all he has to do is up his game.
In Scenario 2, we use the flat xp method. Player A pushes the envelope and pulls off all kinds of epic stuff. Player B grabs a Mountain Dew to go with his Cheetos. Everyone gets the same xp just for showing up and being able to fog a mirror. Player B keeps on keeping on just as always, because why take any chances when the rewards are always the same. Player A sees that it doesn't matter what he does, he will get the same participation trophy. So next session, he sits down and says "pass the Cheetos".
While these examples are to the extreme, this is done to make the point. If everyone at a table is pleased with getting the same rewards no matter the efforts made, then more power to them. In my thirty plus years of gaming, I have never seen this to be the case.
Giving everyone a participation trophy for showing up may make the mothers happy, but the players, or at least the ones with even the smallest amount of drive, think it sucks.
The same feelings tend to apply to the RPG table as well. Players are motivated to succeed and do cool things. The GM rewards these efforts and successes by handing out xp and treasure. Not every group can fill all of the chairs at the table with their preferred player type. Some of us have to take what we can get. That means one player may be Captain America, while the guy next to him is a sponge.
One of the things that I have found to work well to motivate a better and more full participation is the use of individual rewards. This is the carrot that can motivate most players. There are a few players out there who this will not work on. Then the trick is to find what does motivate them.
But if you get stuck with someone who is simply content to sit there and eat Cheetos, then the other players at least get someone to detect traps the hard way and to soak up some crits when they try to Leeroy Jenkins their way into the next room.
It's all good either way.