It all depends on what you are planning.
Not just with spellcasters, but almost any base class, you lose something unique to that class by not taking it 100% (Well, anything that's not a fighter), from spells, to other level dependant abilities, like shapechange, monk unarmed abilites, favored enemy bonuses, extra rages, etc. It all depends on if you feel the gains are worth it.
I've had 2 caster characters who were split classed. One, I brought up from a 2nd level char (3.0 Barb/Cleric), and he split levels till he later went along a druidic path. End result at 18th level, I missed out on higher level spells in both cleric and druid, but I enjoyed the character immensely for the RP value (Barb 4, Cleric 5, Druid 9, and other strangeness thrown in), and given that ultimately the character was more of a meele fighter than a spell caster, it never seemed like a huge loss (Especialy since GM allowed me to add Druid and Cleric levels for effective casting level (but not spell selection/number) since it was a decision based on a vision from my god).
The other, a character I am currently playing, started as a first level Wizard, and branched into Fighter for 1 level so he could become an Elderich Knight. He's an Elven Archer, and will also pick up a few levels of Arcane Archer, though I am unsure of how many. Much of what he has done is for RP reasons (Always wanted to be a noble warior of ledgened, but his parents sent him to train as a wizard, meanwhile, he was excelent with a bow so he sort of melded them togeather). Now, I did feal the loss of the caster level, mostly at 2nd level wizard, but the other combat abilities have helped out tremendously.
So, if for character reasons you miss a few levels, you'll likely not miss them as much because you'll be playing in character. If you're loosing them to "Buff up" your caster, you'll ultimately miss them for what you can do as a pure caster later.
I would avoid taking a level or two in a class specificly for mechanical bonuses without RP reasons to do so. Otherwise, why not take a level or two in monk to get uber saves, some cool bonuses if unarmored (Helps aleviate ambush readyness issues), and get evasion?