[ister]

I'm not so sure that's correct. Even a magic based party can take advantage of various bard songs (immunity to sound based attacks, bonuses to saving throws, regeneration and so on). And the bard is going to be many levels higher than a mage, so it is better to have a bard cast many spells than a mage. The disadvantage relative to a mage is that you get fewer spells, and you get the highest level spells later or not at all. The advantage is that many spells are more effective when cast by a bard, you can sing a variety of useful songs.

A bard may not be quite as useful in a magic based party as it is in a melee based party, but the combination of a bard and a mage is almost always superior to two mages.