Usually the new probe builds just add in support for some new board or product that Asus is introducing. They don't generally change any of the old features, if they do they generally mention it. A good rule of thumb for Asus probe is if the version you got works with what you have, keep it. If it ever starts acting flaky or doesn't do what it's supposed to, upgrade it. :)

One of the more recent releases (I think 2.19.00) added support for the thermal sensing built-in to newer Athlon's (gives more accurate temperatures), apart from that I have no idea what else they change, except to add support for newer mainboards.