I agree with wasting a FP on Illuminations is a mistake, but I just don't see the reason not to in a lot of cases. Depending on the time of year, there is very little need for FP in Epcot, except for TT and Soarin. Living with the land is a total waste, remove it form the conversation. So you are left with TT and Soarin. In Febry when we are going, rumor is that Soarin will be closed for rehab. Going to Epcot for 2 days, we might get TT for both (normally we would do Soarin one day and TT the other), but more than likely we will do TT on one day and Illuminations for the other.
FYI... hidden tip you don't hear much about... if it starts to rain, or even if you are leaving Epcot early for some reason and there is a reasonable chance of rain... Stop at a kiosk and get a TT fast pass for the last hour of the day, they are easy to get often. IF they close TT in the last hour, then you will get an extra fastpass for any attraction at any time the next day in any park. We had terrible weather our last time in Epcot, 2 days of rain. We ended up not doing much in Epcot, but both days got us extra fastpasses that we were able to use to ride SDMT the next day. (This would work in any park, but try to not be disrespectful of those who want the fastpasses and would use them... we were in all day type rain, and there was absolutely no chance that test track would re-open that day, so we did NOT keep anyone from getting the fastpasses, and blocked them out of their legit chance to ride)