It is bad when there is a fixed service charge, e.g. in Pizza Express they automatically add one if more than 8 people are eating, and people "forget" that when calculating "their share". I don't mean leaving a tip is bad, I mean the fact that in groups people often ignore the fact it is often appropriate to leave a tip.
I always say in a loud voice "oooh the service was really good at this end of the table, did you guys really think it was bad enough to withold the tip?"
We all have friends who try and work out their share, round the amount for drinks down (e.g. a £3.95 glass of wine rounded down to £3) completely forget they had 3, rather than the 2 they pay for etc etc...