In our experience, the overwhelming majority of potential employees either have jobs already (and so don't have time to work on contact for you while you determine fit), are getting a job soon (and understandably don't want to turn down a full time position so they can contact for you and maybe get a job later (or not)), or just are offended that you won't commit to them and wasn't too try before you buy (which may be silly, but I admit I felt exactly that way when someone wanted me to do contract to hire).