It's tough to properly justify it - I've had a look at them all (my girlfriend likes them) and found them puerile. Also I worry as a lot of my friends who do like them say that "there are no other books anywhere near as good" (I'm quoting one of them). There really are - even as someone who admittedly criticises them without ever having finished one of them I know that wizards and strange creatures are done brilliantly by Tom Holt, Tom Sharpe does bespectacled nerds doing well for themselves and James Herbert (on his lazy books) manages to do heroes winning out through luck quite well without any help from JK Rowling.
With any luck I'll be proved wrong and my normally non-reading friends who are devouring Harry and his magical chums will go on to read other books. The problem is that I really doubt it....