Greenfield bad: Brownfield good?

I fully understand the arguments for the development of brownfield sites, but I feel they need to be appraised just as carefully as all other sites.   These are some of the problems:

1. The preference for brownfield sites has resulted in the widespread loss of urban gardens;
2. Average costs of developing brownfield sites are high;
3. Average delay in developing brownfield sites is lengthy;
4. Brownfield sites are often seriously polluted, as in the Greenwich Dome site or the Bath Gasometer site.

The public naturally wants old industrial sites to be redeveloped, but the absolute preference for brownfield sites, suitable and unsuitable, seems to have gone too far.