I always bring my external flash to Disney World. However, I usually attach the flash onto the camera only when needed. If I don't need the flash for the next several minutes, I usually take the flash off the camera. It's otherwise WAY too heavy for my neck to be carrying both the camera PLUS the external flash attached on there.
I use the external flash not only for fill flash, but also for the much faster flash recycle times, like others have mentioned. If I'm taking pictures at a character meet-and-greet or during character dining, I can fire off 3 consecutive shots with an external flash (just in case someone blinked, at least I'd have a couple extra photos to choose from). The built-in flash, on the other hand, is too small, not as powerful, and requires time to recycle.
I also hate that my camera's built-in flash will produce a seizure-inducing *strobe* at night. I think it does it to calculate flash exposure and camera focus. And sometimes, it needs to strobe for a few seconds before finally deciding to stop strobing!

On the other hand, the external flash doesn't strobe like that, and I can usually get pretty instantaneous flash photos with it, rather than having to wait for the flash to think about the correct exposure.
I know that others may hate bringing an external flash to Disney World, and others may question whether to use flash at all. But I, personally, feel that some of my daytime pictures are better with a subtle touch of flash, but only in certain situations. And my nighttime and indoor pictures of people are definitely better with an external flash.
The external flash won't help me light Cinderella's Castle, but it will certainly help illuminate my subject standing in front of the Castle.