There is good indication this reduces income disparity, create more opportunities for entrepreneurship (you can quit your job or go part-time while you do your thing) and, in general, more people consume more. The incentive to work more is there: with a sensible basic income you and your family won't starve and you won't need to work yourself to death, but you may want to have a nice vacation from time to time. With better economic security your freedom to move to better paying markets is increased.

The only reason not to back an idea like this is how much the top earners will be able to evade paying their part, overburdening those who can't dodge taxes as efficiently.