For most contracts I suspect that the overhead on a plain English version would be very small, as the lawyers' understanding of the topics is necessarily deep to formulate the contract (or they're just shitty lawyers, another topic).
Plain English versions of contracts, with their plain English meanings of clauses, should be included in any contract between two parties of vastly different bargaining power, i.e. a corporation with loads of legal resources and a non-millionaire potential employee.
Update w.r.t. comments—I understand the points you're making, but I don't think it invalidates the argument. I'm racking my brain to find the examples I've seen, but there do exist in the wild "plain English" versions of contracts that are not binding (and they specify that) but instead contain comprehensible summaries of the salient parts.