Well it's something that DH and I are doing, so obviously I'm for it. Our children are happy and healthy, are deprived of nothing, and get all the nutrients they need.
Yes it requires planning, to ensure they get all the nutrients they need, but surely the same applies to any kind of diet. Children who are raised on an omnivore diet, aren't automatically getting everything they need from their diet, especially if it's full of junk and processed foods.
When DS1 was born, the first health visitor I saw, almost keeled over when I said he'd be raised on a vegetarian diet. I reported her for her comments, as she basically said it was 'a form of child abuse', and that he'd be very unhealthy. Well he's 15 now (16 in December), only ever had one day off school sick, and yes, he's still eating a vegetarian diet (by his own choice now, as he's old enough to decide for himself).
As for the question above, about what happens when they're older, and decide to eat bacon sarnies etc. Well that's their choice. DH and I have been very clear to our children, that when they're old enough, they can decide for themselves. However, our only rule would be, no eating meat in our home. Until they moved out, they'd be free to eat meat at other places (like a friend's home), or at a restaurant, but again, not in our home.
As it stands right now though, that's not an issue, as DS1 (15), DD1 (13), and DD2 (10) are already a self-declared vegetarians. DS2 (also 10) is currently deciding whether he wants to try meat, and we've told him that when he makes a decision, and if he wants to try meat, we'll take him to a restaurant. DD3 (7) and DS3 (4) are too young to decide for themselves at the moment.