'Upgrading' to skinny tyres pointless for London commuting

On the wider subject of tyres (rather than the subject of wider tyres), based on some other current trends, might it be advantageous to run a cheaper/harder compound tyre on the back i.e. your usual favourite on the front, plus the tyre one or two down the brand hierarchy on the back? Wouldn't this be a much cheaper way of achieving the result supposedly offered by the front/rear-specific combo?

This is Michelins thinking with the new Pro Optimum.

They're 25c training tyres with soft/light front and relatively hard rear (still only about 250g I think). Michelin say they should wear at the same rate as each other.