I seem to be in the minority here, while I think the train is OK at Disney World, unlike Disneyland's version, I certainly wouldn't call it essential. I think the big difference is that at MK it doesn't know what it is supposed be:
Is it an attraction? Well, you face forward the whole time and there is no diorama, etc. so, no, not really an attraction in and of itself.
Is it transportation? Well, you only have three stops total compared to the four at Disneyland, given the much larger size of Disney World, I can't really call it transportation either.
That isn't to say I wouldn't ride it if it were open, but at the same time I am not heart broken when it is closed, especially since you can't take ECVs on the MK railroad (which you can at Disneyland).