On the outside, Slime Rain has absolutely no reason to spawn more than once, which is before defeating King Slime in pre-Hardmode. Because of how rare it becomes afterwards, obtaining the music box for it (locked to Hardmode) is very difficult. Not to mention it's a great way to farm rainbow bricks which are hard to obtain otherwise (I think? Feels familiar anyway.)
I also think that there should be a change to Slime Rain that causes Queen Slime to appear rather than King Slime if the player is in the Hallow.