Mike Monteiro has an excellent talk[1] on contract negotiation and where to get lawyers involved. The pertinent point for me was the following:
Contract negotiation makes them fair. If you just sign whatever people put in front of you, you give up your rights to have a fair contract.
Also: The contract given to you probably represents the opposing lawyers wish list of all the things they wish they could have. Most/some of these things will be extremely disadvantageous to you. Just signing anything they give you means you submit to their wish list, not your own.