I think some of them are quite funny (not the rude ones).
I got one ages ago for £250 as a bday present that spelt my name and it looked cute on on my car. It got sold with the car too so someone else liked it ;)
I also give regularly to charity, I don't see how that's got anything to do with it??
I can understand that if they cost grands then you might find it eccentric but there are loads of other things some people buy that are obsolete (dog clothes anyone?).
There's a saying in French:
Les gouts et les couleurs, ca ne se discute pas. (Tastes and colours are not up for discussion. (or something along the lines).
I think it's funny that number plates make you so agitated to be honest :D
Everybody has their pet peeves.
There must be something deeper going on like someone you think is really stupid or arrogant has one or something?