Isn't the main balancing aspect of concentration that is prevents stacking? Not that it's easy to upset with a hit? At least, that's how I saw it.
Well, what the "main" aspect is, is open to debate. You could prevent stacking without any kind of checked Concentration mechanic, couldn't you, SigmaOne? I mean, just say "You can only have one Concentration spell in use at a time". You don't need any checks or anything.
Few (IMO of course) buff-type Concentration spells are good enough to need breaking, either, so they don't really seem balanced on that basis (in fact a lot of them are actively terrible when Concentration is factored in, which kind of encourages you to cast them on your friends but seems a bit extreme).
Whereas the Concentration check DOES balance extremely powerful Save-or-Suck or Save-or-murder-your-friends spells like Dominate. With Hold Person, you save every round anyway, so not a huge deal if you have Concentration or not. Dominate and a lot of other "Save only at the beginning" spells, though? Yeah, the Concentration check is a
big balancing factor there, because it's quite likely anyone who isn't a Cleric (or similar) is going to only have a 5-30% chance to save against those at higher levels (lowest of any edition, generally speaking), but they CAN be broken by breaking the Concentration of the caster.