I think the "logic" behind the advice I received was that there were benefits specified in the contract which I wouldn't be entitled to (such as a notice period? or sick pay?) if I didn't sign it.
Under this theory, I may have been entitled to any benefits that were mentioned during the interview and salary negotiation process, but they might be difficult for me to enforce. For example, if the company had refused to grant those mentioned benefits, as a form of constructive dismissal, I might have had a hard time proving during an employment tribunal which benefits were informally agreed (and were conditions upon which I had accepted their job offer).
