I am a great believer in local airports such as BLK and LPL. I think that every part of the country should be able to access the world from an airport within 30 mins car distance.
BUT, if Manchester is to compete with cities like Glasgow and Birmingham, it needs to strengthen it's worldwide transportation links. The Olympics are arriving in the UK in 2012, now we have all seen the nightmare at MAN when the Champions League Final was held at Old Trafford a few years back. Imagine the airport like that for 2 months! Would drive normal passengers mad, seeing hundreds of extra planes delaying their flights due to eco-friendly restrictions and slot requirements.
If Manchester is to keep upto date with the latest in world transportation, then surely LC airlines need to have more of a presence in the city. Another airport would ease traffic at MAN for long-haul and charter, and the new airport would be able to provide the city with the LC links around Europe, if not further.
About the pollution issue. If a new airport was to be built, it would probably be roughly the same size as LPL or EMA. Both of these airports make less pollution than Runway 3 at Heathrow will, and that has been approved by the Government.