[Twan]

Yes the best system would be the spell ending when the mage leaves, like it was supposed to be (...and a good initiative system instead of defender always cast first).

On the other hand it's a level 8 spell, supposed to be powerful and increasing battle duration may be seen as a part of its interest (9 rounds of any BE + MoD is not really more powerful than other mass spells a defender can cast before the attacker plays - the attacker can use warriors of muspellheim or something to reduce the effects of the second spell, when he can really do nothing to counter instants like rain of stones or mass enslave).