Exclusive spells are good for several reasons.
1. Flavor: Not every spell cast needs to be the same.
2. Variety: In addition to background elements and historical significance in a campaign, having classes with different spells insures that not every spellcaster is played the same. This is good for campaigns with a lot of wizards, but not so important if all you're using is sorcerers, due to the sorcerers limited number of spells known.
3. Game Balance: There may be some other factors involved in the background and flavor of a spell that limits it's use and with those factors removed, it may become more 'common' or accepted.
4. Control: By allowing only certain spells to be used by certain classes, the authors have installed a control device on said spell preventing any one caster from being a do it all.