I generally think any arguments about what people "deserve" to earn go into messy territory fast. The answer to the question is that they deserve a good contract just as much as anyone else of a similar level does.
The thing is, players don't have agents just for fun. They have them because they provide a service that benefits the player financially (even if not in other ways). Maybe they scupper the odd deal, but generally they're hired to provide a personal service and they do just that.
Agents are "bad" for the game in that they drive up contracts and selfishness. But here's the thing, if people think that huge businesses owned by multi-millionaire businessmen aren't going to take advantage of players in negotiation and enforcing contracts then, to me, that's just as naive as anything anyone might say in favour of the agents.
We're talking about big business and big money. I'm trying not to go on a big leftist rant here, but I find it really hard to argue against the idea of workers getting representation to maximise their earnings. If you work for a big company then fuck the idea that the guy who wrote out your contract is looking out for your best interests. They aren't and they never have.
Agents are shit but they're only there because giant for-profit businesses like football clubs aren't going to ignore the huge imbalances of power they have over their employees. This is the capitalist model at work.