Well, the Greens have developed into the most successful left-of-Labour party in England since the last world war. They are now the 4th largest party here with many councillors and a consistently higher percentage of votes than all the "socialist" and "workers" parties put together. And they achieved this independently of any vanguard party and the out-dated Leninist ideology these all have.
The Green Party ticks all the boxes for what any "New Workers Party" would advocate (I'm quote from the policies on which they fought the recent elections in London):
A London Living Wage of £7.20.
Change planning rules so that 60% of all new housing is affordable.
Rail and tube franchises back under public control
Cut all bus and off-speak Tube fares by 20p.
Oppose all airport expansion in London and the South East.
The Green Party also wants to allow local councils to start building housing again, are opposed to privatisation of the NHS and in favour of a tax policy to redistribute income from the rich to the poor, and against Britain's participation in the wars in Iraq and Afghanistan.
In some respects the Greens advocated more radical reforms than the SWP-sponsored Left List (for which dennisr voted if he followed his party's instructions).
So, why the need for another left-of-Labour party when one already exists?
I hope I haven't made the Green Party seem too attractive so that some Trotskyist group decides to "enter" it and bore from within. That would be the kiss of death.
Why don't I join the Green Party myself? One reason is that I'm a motorist and don't fancy having to drive at 20 mph on all side roads.