UBI seems like a good idea but it is important to recognize that the problem it seeks to solve has a cultural component too. Some people will use free money to build enriching lives. Others will fall into various addictions because they haven't found meaning. I'd like to see a UBI conversation that discusses is this point because it is significant and the history of welfare shows that it is real.
First, I am a believer in economic momentum and money multiplier dynamics (perhaps irrationally so). But with this belief more money being spent (consumption) makes for a stronger economy, for everyone, including those of us who are not in need of welfare. Our current welfare system places too much restriction and I feel leaves too many welfare recipients with non-optimal purchasing decision...not to mention there is an overhead to our (America's) current welfare system as far as administration and what-else.
Another thing is that there have been studies done which show that giving cash to the poor has better outcomes than expected: http://www.economist.com/news/international/21588385-giving-...
There are also deeper things we can theorize about regarding poor behavior, but at the end of the day I can't help but feel like a UBI would be a more efficient welfare distribution (more money spent actually issued as monetary welfare benefits) which would have a stronger economic impact than our current welfare system.
Disclaimer: The above is just my opinion, not claiming it to be right or wrong.