There are only a certain type of film that I upgrade from DVD to Blu for...and that is those that benefit from the picture quality. So good examples would be Lord of the Rings, Jurassic Park, the Alien Anthology and Star Wars.
Smaller character driven films I'm not too fussed about upgrading as it is about relationships and not primarily effects. Films like What's Eating Gilbert Grape?, Dead Man's Shoes and This is England.
But then there are classics which are a must buy, simply because of their status in history and as such are worth a purchase on every new format that will ever become available - so that they can be viewed in the best possible quality.
I already own Jaws on VHS and DVD, but still almost pissed myself at the prospect of it's Blu-Ray release later this year!