I agree with Mesaboy in that it's a poor use of a FP+ for multiple reasons.
First, you can see the show for a variety of ways, you just need to show up a bit earlier (think around 8:20 or so for a 9pm IllumiNations showing) and if you really don't want that, you can dine at Rose & Crown or even do the dessert party (trying it late next month so I'll report back!) if you have some cash to burn. The dessert party offers the same (less-crowded) viewing area as one of the FP+ locations (there are two) and R&C offers, in my opinion, an equally as good location.
Another reason is that you then must go standby for Soarin and Test Track. This may not be a big deal if you get there early and only want to ride each once or you have no one in your party interested in riding them, but if you don't care for a rushed rope drop filled with criss-crossing the park, you're going to want a FP+ for one of those.
Finally, the newest and most compelling reason for me not to do the IllumiNations FP+ is that you lose access to that fourth "revolving" FP+ for the rest of the day.