When is Illuminations is busiest/most difficult to find the perfect spot: on a night the park closes at 9:00 on a Saturday, or an Extra Magic night when it closes at 11:00pm? Is it worth it to use a FP+ for this for viewing?
As a good rule of thumb, parks are the most busy during regular hours on their EMH day. Illuminations on an EMH day will have both crowds. If you are willing to save your spot early then it doesn’t matter. If you wish to forgo all other attractions in the FP tier then I believe it is a good use of a FP+.
Like everything in WDW, it is up to you!
I would use your 3 FPP early in the day and try for your 4th to be for illuminations. If you get it you get it if not there are plenty of great spots to view or get there early.