There are environmental costs to having those warm bodies sitting around at home all day, consuming fossil fuels and electricity, causing additional pollution with their waste products while they contrbute zero productivity (as opposed to suboptimal, but non-zero productivity by having them carry out 'bullshit make-work').
No. Stop. You have this wholly wrong. It is better for the world if they sit at home and watch American Gladiators than if they go to some bullshit job and do something we don't actually need them to do, which consumes even more energy than staying at home and getting dumber.
If we are to measure the worth of people's existence by their net contribution/cost on the biosphere, do you really want to take this line of logic to its inevitable conclusion?
Yes, because I have actually thought this through, like you clearly haven't. Have you ever read the Mars trilogy? It spends a lot more time on eco-economics than I reasonably could in a Slashdot post.
At least while working, some will have the opportunities and incentive to become more productive.
UBI doesn't permit you to live high on the hog, or at least that's not the concept. If you want more than a banal, minimal existence, you're going to have to work for it. There is still plenty of incentive to produce, and there's still plenty of opportunity to do so as well. It frees people up to do neutral or negative things, certainly, but it also frees them to do positive things. They can spend their time making themselves better people, or doing things for other people. They can trade labor, or engage in barter, or work a side job to get spending money.
I am not against work. Work is how things get done. I am against senseless work which wastes energy and materials whose production costs are debited from our collective future. The notion that our value as a being is based on what we do for society is not without merit, but we must subtract what we do to society from the balance in order to come to a fair accounting.