Cost is one thing, most people won't notice a 10% difference per year on fuel so cheap anyway, but as far as C02 targets go, we are all (apart from the US) required to slash them significantly, I just don't think 1 MPG will make a big difference.
I hate diesel-gate for giving diesels a bad name, recently some car reviewers tested a new Mercedes-Benz wagon and got almost 80 MPG without trying (76.3 to be exact) an older one would have struggled to hit 40 MPG.