[Saros]

At optimal 20-th druid/priest level, Barkskin caps at setting base AC to 1 - same as Spirit Armor. However, Spirit Armor, even cast by a 7th or 8th level wizard, still sets AC to 1 and grants 3 bonus to saving throw vs spells. So, Spirit armor is the better spell here.

As for 'pro' multi-class: I would add just one, and the most important one IMHO: combining all of the subclasses' HLAs, and getting much more HLAs than a dual-class. This is the actual strength of a multiclass, since combining certain different-class HLAs is extremely powerful: like combining UAI and Time Traps with warrior's THAC0, GWW, Critical or Hardiness, for instance.