The reason Catholics leave ashes on their head is to be a witness to the faith. Not to show that they are doing something extra-ordinary to gain sympathy, which would be like fasting before men.
Not to elevate themselves above other people but just to show that Christianity does exist and maybe inspire someone a little.
God Bless
Scylla
(I have to admit I am guilty of wiping my ashes off, in fear of someone knowing I was Catholic. This is before I knew anything about how precious my faith is.)