I think Saria is more flexible than Exu since healer defenders double as, well both healers and tanks, which is almost always useful. Exu will not be great against high DEF enemies, where a single-target caster may be more suitable. I still use both in my team tho often, despite having all the 6*s.

I love his squad names, esp on his CN account (I plopped a TL in the alt text):