Balanced! If you multi-class you are essentially trying to be a bard (Jack of all trades, master of none). Accept your fate and know your role - support.
A multi-classed character will always be "less powerful" than a single classed character, but they have spread-loaded their skills and feat to be more optimal in different situations than others of their ilk. Since mages seems to be the focus, lets look at the trade-offs, as a magic using type, you have phenominal cosmic power and the vitality of an impotent snail, should a lucky shot with a bow get through, you're toast, even at higher levels. Any class you take should give you a higher "survivability" factor when things get messy. You trade power for pain soak, simple as that.
The reverse is true for other characters, you sacrifice the killing stroke (fighting type), defensive post (priest types) and skills a plenty (rogue types) for that chance to have the right spell at the right time. Spider Climb for the thief that absolutely, positively has to make that climb check (or Knock when the door has to be opened or your money back, usually at sword point). Bless, Aid, etc is great, but if you can't deal damage, what good is protecting yourself from it. And imagine the surprise when the approaching enemy prepares to attack that Magic Missile slinging mage only to find out that he has armor and weapons outside of the class mold as well as being buffed to high heaven. (pun intended)
And what about the 'naked' fighter, that happens to have Mage Armor and the high Con and Dex bonuses and can still use a shield or conjure one up as the case may be. Imagine the look on the opposing wizard's face when the Magic Missiles he sends at the charging fighter glance helplessly off the Shield spell, right before the fighter hacks off his head. Utility is the watch word, not "I can't cast this even though I'm "X" character level." No, spell casters aren't hosed, they are on par with the rest of us "mere mortals",