> 1) Why would I waste the candidate or the client/employer's time if there's too large a gap between candidate's salary & their highest potential for this role?

It's trivial to solve this problem for a recruiter by simply stating the company budget range upfront. The problem is that recruiters want the client to give up their information without revealing any of their. That is where the asymmetry of information come into play and engineers end up getting the short end of the negotiation.