If a potential client wants a plan in advance, we agree to a project fee to cover time and effort. Otherwise, it's best to refer to references and clients and leave it at that. I have found out more than once (the hard way) that your good work suddenly gets done "in house," with an incumbent or elsewhere -- and the worse for you when you don't get paid and yet your ideas are put in play. Unfortunately, some individuals act unethically.
I still find it astounding that p.r. and ad folks are asked to do actual client work on spec or as part of the sales process. If a client can't judge you by your past work and a sense of "fit" from the interview process, they aren't competent. This kind of thing is at the level of local radio ad sales where I would make spec spots because store owners would buy based on a spot moreso than whether the medium was correct. Are businesses big enough to hire ad and p.r. firms no more sophisticated than the pizza shop or auto repair shop guys I used to try and sell?