I tend to think if you have to ask you don't need them, but in seriousness,

Pros:
- Can set corner weights precisely
- Easier to change spring lengths and rates
- More wheel clearance
- More sikness

Cons:
- Price??
- Not really much

If you are going to use an XT130 strut and standard perches you'll need a custom wound spring. The XT struts have a big lower perch, and the upper hat is smaller on the AE86, so you need a spring with a big bottom and small top. I had a set done at Browns Springs in Melbourne, but I'm sure someone local could make you a set if you don't go coilover.