xp is the only currency in the game. the stat was the only indication of efficiency. The bar was pretty low, but I feel like it was the most useful stat they gave you.
Like seige and hero damage are gonna fluctuate based on all kinds of random shit that you don't really have control over a lot of the time, and they're not a very valuable indicator of how well you're doing at any specific point in time. Like hero and siege damage will fluctuate based on team comps, talent choices, and map... it takes a lot of critical thinking to determine how much effect your level 7 talent choice has on your damage output between two matches where literally every other variable can be different. Hero/seige damage is most usefull when there's another player playing the same hero on the enemy team... then you can ask: why is the enemy KT doing 40k more damage then our KT? and have a reasonable expectation of coming up with a valid answer. The xp stat is understandable like this all the time.
xp contribute doesn't care about your talent choices or team comp, and you can always compare it against every other player in the game, and it's generally consistant based on map and hero choice. Like you can make a video on xp mechanics and it applies to everyone pretty much the same in almost any situation:
https://www.youtube.com/watch?v=CJ22EC_cJYE&index=4&list=PLRz2JWFFHXPfp7U6spkl8K1HyM8K2dTC0. Which isn't true for the damage stats.
Now, it's kinda totally misleading when the friendly raynor thinks he's doing well because he's got high xp contribute.... but at least it tells you a very specific thing about what's going on. Like, you know right away it's because he's been in lane the whole game and doesn't come help in team fights.
But if you want to analyze his damage numbers you gotta think: Is it his talent choices? is it the map? is it the enemy team comp? does the other team just suck? or is this raynor just really damn good? but it's probably some weird complicated combination of all that.
xp was the best stat because it was the least obscure once you understood how xp works in the game.
edit:
You can tell it's the real Kirblar because he looks pissed.