If you were to further lower the costs of labour, you risk stymieing actual economic growth which comes from improvements in efficiency. For instance, why use a machine when you can just hire more people?
Also what would the effect on monetary policy be? If this was funded though an income tax, would the lower savings rates of the poor, put more money in circulation, thus increasing inflation? Could you account for the change in prices driven by an increase in demand from those who's labour isn't actually valuable enough to sustain their consumption.
I don't claim to have the answers to welfare, but I really think that we should aim for a solution that provides as little market distortion as possible. I sincerely think that a negative tax would be a huge unwelcome and ineffective distortion in the labour market.
