There is little doubt that greater runway length is going to provide operators with more flexibility. It's all very well if airliner "x" can get to city "y" off 2500m of concrete, in still air at sea level, but given that most airports aren't at sea level, wind conditions aren't ideal, and temperatures are often high, what is theoretically possible is often impossible in the reality of daily operations.
BHX runway extension is therefore going to be at least beneficial to the current operators, and will give the airport the opportunity to extend it's horizons.
Look across the channel; most major airports (serving the biggest cities / conurbations) have 3000m plus runways. In the UK, sadly, this is not the case. Apart from for London, the major cities (Birmingham, Liverpool, Newcastle, Glasgow, Edinburgh, Cardiff, Belfast) should all have this basic facility - sadly currently - and I'm writing this from memory - only MAN and CWL qualify, with BHX joining the 3000m club next year.
It's the "infrastructure thing" that has held the UK back for years that has resulted in this lamentable situation.