Everyone occasionally derps on clerics.
Sometimes when your last cast was a non-instant GCD spell and you pressed Clerics to early and it didn't que.
Sometimes you think clerics didn't que and then press it to find out that it did and you just re-entered clerics for 5 secs (rip tank).
Mistakes happen, wipes happen, but just because a wipe can happen doesn't mean a healer shouldn't try and DPS. If you were to take the amount of time saved over the career of a healer who DPS even with the occasional wipe over one who doesn't, the one who DPSs is still going to clear stuff faster.
