I understand that the removal of the barriers is to make the road more pedestrian and cyclist friendly. As I understand it the thinking (backed up by research from across Europe) is that if the road is open it makes it less easy for drivers as they have to watch out for random pedestrians crossing. They therefore have to take more care in observing the road, this in turn makes them drive more safely and not speed. Roads with railings = roads safe for speeding and bad driving.
Before when the barriers were in place drivers knew that as long as they stopped at the pedestrian crossing there was little chance of anyone else being on the road so they could drive through Brixton centre with little thought for the pedestrians and cyclists.
The railings also made it more dangerous for cyclists who could be squashed between the vehicle and the railings if a lorry or other vehicle did not see them whereas now they have a bit more chance of coming off less injured as they will be able to fall onto the pavement.
As someone else mentioned this is the idea behind Kensington High Stret modifications which I read recently have been considered to be a success in making the roads more people friendly - I read recently that cyclists make up 25% of the traffic on Kensington High Street now and this article
http://www.guardian.co.uk/society/2006/apr/12/communities.guardiansocietysupplement cites figures that show that the number of accidents has gone down.
We can hope that once completed the modifications will calm the traffic on Brixton Road making it more people friendly.