That's where signing with the right agent is so important. Agents negotiate and vet publishing contracts on behalf of their authors so writers need to sign with an agent which they feel will fight their corner and negotiate the best deal possible as well as advise them when they receive multiple offers.
True - but agents are not as independent as you might think and often have a vested interest in keeping the publisher happier than the author in the negotiation of the contract, as Kristine Kathyrn Rusch points out in her excellent book Closing the Deal (she also detests that agents now have contracts with their authors; in the past they didn't).
Rusch suggests you get the agent to get the deal, fine, but then get a publishing attorney to vet the publishing contract - if you can afford one. The alternative is to get an organisation such as Society of Authors/Authors Guild to vet a contract.
But back to your point, yes, if you get the "right agent" you shouldn't have to worry about them short changing you.
Why pay a lawyer to review publishing contracts when you already pay your agent 15% or 20% commission just to do that. Of course agents are dependent on the publishers but also the more money their author gets the bigger their own commission so it's in their interest to get you the best deal possible. Again it goes back to why it's so important to sign with the right agent, to do your homework before signing with them.
I definitely agree that if you don't have an agent then use the Society of Authors/Authors Guild to vet a contract.