You ask a good question. I've always been of the same mind....why would I use a valuable tier 1 FP+ for something like Illuminations? There are tons of great places I can see Illuminations around the World Showcase, without burning my tier 1 FP+. Give up a ride on Soarin' or Test Track to see a show I can see just fine without FP+.....no thank you.
While I still think that may be generally true for most people most of the time, there are times when it might make sense to try it. The previous poster's example is a good one. For us it was New Year's Eve. Here is the general benefit to an Illumination FP+. You won't have to stake out a spot as early, you will have a better view, and you most likely won't have to worry about some selfish (bleep) and his annoying (bleeping) family muscling into a spot in which they can't fit right in front of your kids at the last minute, blocking the great view you HAD from the spot you staked out 30 minutes ago or more. I'd imagine same holds true for parades and fireworks in the MK. The view from the Illuminations FP+ viewing area is also fantastic, best I've ever seen. You feel like you are underneath some of the big shells, where I haven't experienced that out on the Promenade.
I have seen reports by people who have had very crowded experiences in the Illuminations FP+ viewing area, so results may vary I guess.
We loved it and had a great experience. As for our experience on NYE, having FP+ for Soarin' and Test Track earlier in the trip, and knowing how extremely crowded Epcot would be on NYE, I snagged the FP+ for the 6pm Illuminations. Probably the best FP+ I've ever made. We strolled into the FP+ area around 5:15, where the FP+ area was uncrowded and sane, a far cry from what was happening outside the ropes. Great view right up front, room to stretch out and breath, fun banter with CMs in the area and some other lovely guests. Outside of our little FP+ cocoon there were people who had been waiting in spots literally for hours, pack in like sardines, with marginal views of the show. I kid you not. We grabbed a spot right by the rope that separated the FP+ viewing area from some general viewing space in the area. The people on the other side of the rope had staked out their primo spots probably 3 hours ago, and they were packed in like cattle. Moooooo. Some of the crowd, packed in amongst the cranky kids, BO, all variety of unappealing looking half eaten food, barely room to shift their weight to the other butt cheek.....well, they started jumping the rope into our little oasis of FP+ sanity. I kid you not. I felt like a VIP as the lovely CMs kept the non-FP 'riff-raff' at bay, beating them back into Illuminations hell on their side of the rope! I'm not sure I will ever do Illuminations FP+ on a regular night, but it sure was great on NYE.