An ophidian in the one stack not only protects that stack from offensive magic (except for tremor spell, that still worked) but if you target a non-ophidian army with a multi-square AoE spell such as starfall, and the starfall AoE would include the ophidian-containing army, the starfall spell can't be cast on the non-ophidian army. So it's not just the same army being protected.
The non-ophidian armies are subject to single-square AoE spells of course.
I can see how some argue it's a feature, while others argue it's a bug.
In tactical combat, will an ophidian prevent all offensive spells being cast on its fellow units? Memory (which is not always reliable) tells me 'no'.
I'll argue that it should be the same in tactical and strategic combat -- if an ophidian protects all its fellow units from offensive spells in one it should also in the other.
If this is a feature, then it makes the ophidian arguably the most powerful unit in the game (if you rely on magic anyhoo, and this is the 'War of Magic' game series...).
If this remains a feature I foresee a suicide mounted melee strike force to kill off the ophidian unit, then retreat to deal with the remaining forces 'normally'.