@Milk A lot of people do that, I also see a lot of people rather play by themselves and leave one party uneven (with randoms) I don't know why you wouldn't want the booster unless you didn't want to deal with being auto retried if host was inexperienced or prick to get that last retry.
The major issue is, if your going to drop out after the party has started, even 5-10min in. Why bother leaving team "handicapped" when someone else who was willing to stay could have easily replaced you. It isn't fair to the team trying to complete objective if you aren't prepared to stay. If it's not bad enough they died, and dropped the grade, you either have to push through with 1 less player or abandoned and retry before event is over. It only get's harder from here, and this type of behavior will only make it much more difficult.