While it may be true that higher level casters should have an easier time doing what they do, higher level fighters (or the half-ogre that I was talking about in the previous example, who hates mages and rushes straight through everyone (including blade barriers...which has become the enemy's favorite tactic in this campaign...those things sure do hurt sometimes, and I sure am damn glad to have those 250hp to help soak up that damage) to get to them and smash them as soon as he can) have also been doing what they do for a long time, so they should know how to disrupt spells. It has always been my view that spellcasting shouldn't be done in combat, though I would make exceptions for certain spells, but only very rarely (maybe burning hands, or teleport without error or something like that). It's not hard at all to keep your mage out of combat...in 12 levels, I can't remember my sorceress ever having to cast defensively (or even make a concentration check, for that matter), because I would stay out of combat, where the mages should be in the first place. The liability of being a mage and having tons of magical power is that you are in big trouble as soon as someone gets near you...as long as you paly it smart, though (and have teammates who will help you keep the bad guys away from you), you shouldn't have to worry about that.
I have little sympathy for the cleric in your example, though, because he could just as easily cast the spell when far away from melee and walk in, holding the charge, and touch his friend (which wouldn't provoke an attack of opportunity or anything), or he could also stand 5 feet back (out of the bad guy's reach as long as he's medium size, or at least having his friend between the bad guy and himself) and cast the spell, touching his friend and then getting out of there. Of course, clerics also have heavy armor proficiency, d8 hd and medium bab progression for a reason: they're supposed to be taking a lot of hits and such from healing on the front lines, whereas the mages, with their lower hd/lower bab/arcane spell failure should be sticking back and not getting into combat, so there should be no need for them to cast defensively or anything.
Er, I'm sure that rambled. But hopefully you get the drift. I hate concentration ;-)