I think a definition of something can be useful and a timesaver, as long as there are enough people that know what the word means. Saying one is pescatarian or lacto-ovo vegetarian only works if the other person knows what that means.
Kaplods - you bring up a good point of 'vegetarians versus normal'.
In a lot of places I'm sure that idea still exists, but overall the numbers of vegetarians and eco-awareness of food choices are rising, and 'labels' or definitions are becoming handier. In my opinion.