for what my humble opinion is worth, I do not think they are obscene, people spend far too much money on many other things which i find much more obscene.
What irritates me however, is that many of these so called personalised numberplates are actually ILLEGAL, due to the messing around with the spaces and the cutesy scripts they get written in, to make numbers look more like letters and vice versa. Those numberplates are there to help ID a car not to entertain other road users or as a status symbol.