This kind of scenario only happens when people are trying to deliberately snipe others to squeeze in some healing, since there's probably nothing to do from either a) overgearing the content, b) excelling at the fight due to experience, or c) combination of both.
Not the case. This also happens when a Tank freaks out over sudden heavy tank damage, blows his heal, when a healer is half way through a GHeal Cast. Is OH 100% avoidable? Nope. Is it bad? Only when mana comes into play. Is it something that should be considered when trying to become a more efficient healer? Of course!
For example, when doing the alt raid and doing heroic spine, we had a holy priest that was barely doing any healing while also going OOM. I went through skada to see what spells he was using, and watched him during one of our attempts while the damage was high, and realized he was just using the incorrect spells for the damage. OH did not tell me this, looking deeper into the issue is what did.
It might not tell you in that specific situation, but it very well does in others. As a matter of fact, OH numbers (by spell) could potentially save you from having to give some huge explanation as to how you can help fix spell selection errors, simply by telling you exactly which ones (particularly medium to large heals. Not so much HoT's) are being used incorrectly, and how much mana was wasted, using them. If anything, it could make your spell selection explanation easier.
Quick, simple, respectable answers that could point a Healer in the right direction, so that they may work on ridding themselves of bad habits.
If you're not trying to help a healer, of course, the OH stat is a complete waste of time.