The idea
The arguments are well worn, but to me it boils down to:
Cannabis is not so harmful, that it is reasonable to impose the kind of legal penalties on it that currently exist in the UK
The BBC summarise the key points here ( http://news.bbc.co.uk/1/hi/uk/693257.stm )
Why is it important?
I think that most people would agree that this would be a significant measure.