I'm sure that sometimes, lag is certainly just an excuse if the player is embarrassed to admit they had made a mistake. With that being said though, depending on the situation a second or two of lag when healing can be devastating and as a result more easily noticeable that, at the very least "something undesirable happened".
A dps class lagging for a second or two, in most cases will go unnoticed.
Others have suggested that spamming keys to heal can cause you to disconnect, but that doesn't explain why players who spam attacks much more quickly than heals don't disconnect too.
Personally, I like player Viniculus's response:
Blaming lag is more polite than telling you "no, I'm not going to heal your pet."
I agree with Eyonix, but what do you think? Do you think healers lag more often than the rest of the group? Is it just more noticeable because people die? Or is it just being used as an excuse?