That is the absolute first time I've heard of Caelum being the 'province defense killers'. There are alot of problems with Province defense and it's feasibility/usability vs Castles, but Seraph are not top on the list. Any point after turn 20 in the game there are so many things that can kill high costing PD that Seraphs are the least of your worries.

Anything with Raise Skeletons, quickness/frozen heart, false horror, blade wind can clear most PD without much effort. The main problem with PD is that they are so vulnerable to even mini-combatants.

Caelum is more than likely Banned in most of those games, because of the new influx of Map Designers, and their tendancy to overterrain maps. Causing nearly all nations to be at a strategic disadvantage and Air being more overwhelming than it already is. In such instances of heavy terrain flagged maps, flying is king and Caelum is king of flying and Air Gems for summons that fly and flying boots.