There's just no point to that from the employer's or your point of view. If they decide youre not getting on well enough they can dismiss you anyway, contract or no contract, within the first 12 months. Likewise, you can give notice and leave at anytime too.
If you do want to join the company, you could always write them a letter stating what youre agreeing to in the way of salary, holidays etc. Then there's no argument, not about leaving, but about holiday entitlement, how much they will pay you etc.