Hm. I think perhaps a better way of putting it is that the hard AND issue is when people search using a natural language type query (I know about stop words, assume these are always filtered out) and include some extraneous term, so 'What is that new crime novel by John Grisham about a Penguin I think?' will return nothing, and no amount of extra terms added at the end will help, until you delete 'Penguin'... Of course it's anecdotal, but I still suspect it's one of the reasons for the hard AND to OR switch...