Our group ended up doing this, at least as far as PCs went. Our spellcasters were a sorcerer and favored soul - the player started as a cleric, but switched because he didn't like prepping spells.
As someone mentioned above, there can be advantages to having spontaneous casters. If you do want a given spell that they know, you can have tons of it on demand. Whole party invisibilities, D-door hopping without typically sacrificing it as an anti-grapple, etc. You can really see the difference at level 12, when suddenly the favored soul has potentially 6 Heals. Sure, a cleric can also prep all Heals - but there are other good level 6 spells that you probably don't want to lose out on either. A favored soul will have exactly as many Heals as the situation requires and no more, so he's free to use the 2 spells he knows. Same thing for protective spells - especially if something gets dispeled.
And, I think that the versatility for wizards is slightly overstated. A wizard who acquires no new spells barely knows more than the sorcerer, and has to prep in advance. So a wizard needs to buy/find scrolls to have a fat spellbook. But if he can get scrolls, a sorcerer can too. And the sorcerer can often just use utility scrolls when needed to cover situations outside his known spell spells. So between scrolls and a decent initial spell selection - for the love of god, DON'T PICK A THEME, a theme -> redundancy in your spells; just pick good ones - a sorcerer can do a job covering the arcane caster role.
The roughest part in that game was that our Favored Soul didn't have Divination or Commune, so our strategic information gathering ability sucked. He actually had them at one point, and then switched out because he didn't like talking to the divine intermediary that answered his questions.