It would indeed be more efficient (fewer distortions to earn income) to replace the current systems in place in almost all countries. These systems' stated goals are to provide a minimum acceptable standard of living to everyone. But, the systems involve multiple transfer schemes, programs with conflicting incentives etc, usually making it worse for people on both sides of the equation. Money is money whether you receive it through, say, a food stamp debit card, or through a housing voucher, or a school voucher. However, the fact that it is tied to consumption of specific goods prevent the poor from achieving all that they can achieve with the same amount of resources.
Therefore, I could support a system that guaranteed, say $25,000/year if it is coupled with the elimination of all the not so itty bitty labyrinth of targeted programs. Unfortunately, targeted programs that benefit small groups at the expense of larger ones are the bread and butter of politicians.
Philosophical support for things like basic income guarantees can be traced to Rawls' Theory of Justice whose implication is the maximization of the welfare of the worst off. While it sounds nice in the abstract, let's look at how Rawls arrive at this:
They are the principles that free and rational persons concerned to
further their own interests would accept in an initial position of
equality as defining the fundamental terms of their association. (emphasis mine).
That is, he envisions a hypothetical state where you and I are going to be given some cake from somewhere, and says the only fair and just thing is to assume that we have equal rights to this cake and takes off from there.
His work is all well and good as a consistent logical system. In the real world we have a slightly different situation.
In the real world, you and I each make our own cake, buying our ingredients, putting in time etc. Due to a combination of luck, skill, and effort, we produce our cakes. Let's say my cake is small and tastes bad and yours is ample and delicious. Applied to this situation, inappropriately I might add, the principle implies that part of your cake ought to be taken away from you and given to me.
I don't mean to imply that a nice person who shares her cake with her neighbor is doing anything bad. That would be a nice thing to do. But, distributive justice does not concern itself with what is nice. It gives rights to other people to consume the cake you made.
True, your bounty had something to do with luck. But, it also had something to do with skill (a combination of luck in natural endowments and past effort) and actual effort.
In a society where if you don't have cake, you are guaranteed a share of everyone else's, regardless of what effort you put in, there is an adverse incentive to making cake.
In general work is a bad, and leisure is a good. If it were otherwise, you wouldn't have to be paid to work. Everyone would prefer to get their current income without working to getting the same income by having to work. By not having to work, you wouldn't be completely idle, but you'd engage in activities that you find enjoyable but are probably not as highly valued as the goods and services you provide other people.
That is, there would be less cake to redistribute.
All that aside, I would still be in favor of a system that guaranteed a basic total income without multiple transfer schemes in place, because the complexity of the current system leads to an inefficient allocation of the resources of the poor and the rich alike.
Such a level of income is best achieved through a single lump sum transfer and without an individual's earnings affecting the level of the transfer. Things like minimum wage laws prevent unskilled workers from gaining experience by locking them out of the labor market.
Other examples abound, but this entry is already getting long.