Why is the infield fly rule such a big deal when there only seems to be one scenario when the infield fly rule can be in effect, and then it seems like it's a totally subjective call - like if the umpire feels like it or not. So why do they have it anyway? Why not just get rid of it? It seem so arbitrary. And confusing. Like a throwaway rule or something, take it or leave it. Bizarre.
Perhaps I was not as clear as I had hoped to be. You're right in one way - the infield fly rule pretty much only applies in one scenario, but it is a scenario that happens often enough that it is an issue.
The reason why it exists is to stop a force play - if the infielder drops the ball and the bases are loaded (or there are runners on first and second) then the runners have to move to the next base creating an easy double play for the defensive team, ending the inning if there is one out already. Instead of one easy out - where, if the infielder had played it straight the other guys wouldn't have even tried to run - they force the offensive team to move when they otherwise wouldn't have and the defense gets two easy outs.
It's about trying to keep things fair. Because, if the batter had hit a fly ball out to center field, the ball would (usually) be caught and the runners already on base would have stayed put. (They can tag up and try to advance but the chances are slim). When the ball doesn't even leave the infield, then the runners don't really stand a chance.
The reason why it is a judgement call by the ump is because the someone has to have ultimate authority, and for this kind of rule, a lot of scenarios could be argued that really don't apply.
I hope that cleared things up. If not, just pretend it doesn't exist. But don't come complaining to me when your little leaguer is picked off at second when they should have been safe. ;)