Back when I first joined in 2008 and for a couple of years afterwards, it used to be that way. You'd get an Xper point for every up vote received on your answer which led to a lot of nonsense shit being posted just so you could farm xper points. I don't think you got one for up voting other answers though.
That'd be cool but then I think people would just start up voting their friends. I have a lot of friends here and while I'd appreciate them up voting because they like me I'd much rather have them up vote for the actual opinion.
I mean, this sounds like a communism-- a great idea if it wasn't going to be abused and corrupted. (No, not like communism in a literal sense.)
Some potential problems --Questions that xper farm. "Hey make an opinion on my question and everyone upvote each other?" Or those "See who can get the most upvotes?" --People that xper farm with themself. People that make multiple accounts and upvote their own opinions. --People that upvote everything. I'm all for free points, but there is no limit on upvoting. This would lead to... --Xper 'inflation.' Suddenly xper wouldn't be worth as much and GAG stock would go down. Amazon giftcard prices would skyrocket since it'd be easier to get xD --Those people that post something stupid like "Upvote me. For no reason at all *insert picture of Peter Griffin." Or "Upvote me and I'll upvote you!"
Suddenly xper wouldn't show someone's dedication to this site very much. The system could be even more easily exploited with +1 upvotes. It seems like a good idea in theory, but a bad idea in practice.
The most useful opinions would mean next to nothing, and the popular users would have their followers upvoting every opinion they gave (and they would start giving an opinion to just about everything). So if a person was popular, the could make master in a couple of weeks without ever giving helpful advice. Just answer an opinion with "Hey guys, it me." and get dozens of point on every opinion.