I am pro R3 because I see LHR as incredibly constrained and the third runway is merely a reaction to the increased demand for flights into the capital as well as giving UK airlines the platform to generate business away from their home markets using the hub and spoke system.
However, I try to take a balanced view of things and I can see where the anti-R3 people are coming from. The bottom line is that this country is far too London-centric in terms of the way business is conducted. The government should be trying to diversify and encourage businesses to the regions to spread wealth across the whole of the UK (especially during this digital age where location isn't as important). If they do that, then existing runway capacity can be used more effectively.
Also, as LHR is a hub and spoke operation, you need the feeder flights into the airport to justify the VLAs operating to all corners of the globe. There simply isn't the demand to operate to many destinations from the UK regional airports without feeder traffic from across Europe. Point to point Euro flights are ok with 150 seats. Filling 300-400 seats for a long haul flight requires feed. Just look at EK. This is why we only see a significant number of widebody long haul flights from Heathrow.
If we assume that the country will weight itself more and more towards London, then LHR R3 needs to be built. Simple.
While they're at it, LGW should get another runway anyway to future-proof it and enable dual runway ops. Bulldozing the CAA would be a nice start
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Champ