Early in the hiring season, top-tier candidates begin getting offers from mid- to upper-level schools. Often, such schools have a ranked list of candidates to whom their deans are authorized to make offers. If a school's faculty has authorized the dean to make offers to, say, eight candidates to fill two slots, two offers will generally go out -- three if the dean has some financial flexibility and could live with the unexpected good luck of three acceptances.
Those slots are then out of play for the rest of the candidate field until the initial offerees make up their minds. The other six candidates the faculty has approved must sit around wondering what will happen next and when. They may face pressures from other schools lower on their preference list. They may need to begin planning to move their families. Too bad. They must wait until the initial offerees run the clock out -- as often happens.
Much of the commentary I've read about deadlines focuses on the needs of initial offerees -- typically the most highly credentialed candidates. But such candidates constitute only a small part of the entry-level pool.
Two practices create the problem to which expiring offers are a solution.
First, highly credentialed candidates commonly stockpile offers. Second, top law schools often expect candidates to wait around until mid- to late spring. Both practices inconvenience everyone else. The rest of us need the stockpiled slots released as quickly as possible so everyone else can get on with the process of finding an academic home. Short deadlines unclog the system.
Focusing solely on the "evil" of short deadlines assumes that stockpiling by highly credentialed candidates and mid- to late spring offers from top schools are themselves unproblematic. Even the language commonly used is loaded. Calling offers with short deadlines "exploding offers" is a lot like calling the estate tax the "death tax" -- it presupposes a particular normative outcome.
The simplest solution is already within candidates' control. Candidates who are concerned about short deadlines should ask about the offer policies of the back-up schools at which they are interviewing. They shouldn't interview at back-up schools to whose policies they object, take up offer slots that other candidates really want, and then complain about the deadlines. If enough top-tier candidates were to use as back-ups only schools willing to leave offers open for extended periods, such schools would presumably get more and better candidates. Schools that wanted to finish their entry-level hiring expeditiously wouldn't find their offer slots clogged by candidates who don't really want to teach there anyhow. Ultimately, the complaint about short deadlines is an assertion that all schools should be willing to serve as back-ups -- a premise with which one can reasonably disagree.
I agree that hardball tactics for the purpose of putting a candidate in an awkward position are reprehensible and counterproductive. But the issue of hardball tactics is analytically distinct from that of offer deadlines. The fact that some deans misuse offer deadlines does not mean that such deadlines -- even if short -- are themselves illegitimate. The contrary is in fact true: deadlines make the system work.