It's all about covering the greatest area in terms of range, keeping the characters with least HP and AC out of harm's way, while being able to deal damage to enemies at all ranges.
If you have a spell caster heavy party, you're going to find you'll get overwhelmed quickly and die faster due to the lower HP and AC of Spellcasters.
Also spells have a higher chance of failure given possible broken concentration etc. Which means you need to protect your spell casters if you want to be most effective in battle.
