It depends on the scene.

I come from a live music scene (used to be a local promoter and booker for metal bands in my city), and so in that instance, it's real or nothing.

BUT.

Since getting into k-pop, I've come to understand that there are circumstances where syncing is perfectly acceptable. Usually when a Korean pop group perform, they have lots of dance movements, with songs having full choreography and such. In this case, they might have a backing track playing, while also having the microphone - so they do their best to sing as much as they can, sometimes the singers in the background will fill in the lines while the front of the groups are dancing, and then they switch positions etc, but occasionally when the whole group is involved in particular dance moves (usually during the chorus) - they concentrate more on the dancing than the singing. And it's perfectly acceptable, many groups do this and many fans understand that it's virtually impossible to dance all the moves in the music videos that they love while also singing every single line perfectly. So they try to do a comprimise in order to deliver both as best they possibly can. Some Korean groups don't do this, or will only have an instrumental track in the background with no vocals, but many of them do.

So I think it's okay in that particular instance.

I understand it at something like the Superbowl, where there's a huge production number with lots of "dancing," etc., but why would I want to see/hear it? It's just a lot of hype.

I agree with D: I may as well listen to a recording as to lypsynching.

I think miming or lip syncing once in a while (ie sore throat but not wanting to let fans down) is ok.

But to do it all the time is just plain rude and insulting to fans