BHD,
It was an unfortunate situation your son may have been in to cause so many turnovers, but I am not sure that is always the case.
I think that most players know exactly where they stand and it's not hard to figure out what the coach wants, the best players play. But some of the best players don't get to play right away, there is a right of passage at some programs.
My son started and ended with all of the players who came as freshman when he did, a few 2004 HS grad JUCO transfers came in to replace older drafted players that decided to sign, no one was ever cut. If you left it was on your own. I don't know about other programs, but it seemed to me, in the conference he played he faced the same core of players from other teams, year after year.
I don't want parents of future college players to stay awake at night trying to figure this all out, for most players it works out, most scholarship players do not get cut at most programs.
My son is a pitcher so we didn't have to watch numbers, and position players shouldn't always have to either, many coaches recruit players based on ability and many position guys do not always end up where they began or what they came in for or where you played in HS, that is why being versatile for position guys is important and being able to be a consisitant hitter. If you stick to one position, and you can't adapt to others, there is a reason you will sit. The exception may be catchers.
I would however recommend noting if the core of a team changes from year to year and a player deciding to walk on should always pay attention to fall roster numbers.