So let's say that Joe Sixpack can't get a "real" job. Then society's choices for him are
1. Let him starve to death. Hope that if his alternative is starving then he will reform himself enough to get a job.
2. Give him money with no strings attached (currently various welfare programs, possibly UBI in the future)
3. Give him money but make him work a gov't job where he subtracts value (e.g. he is actively making things worse than if he was just not there)
4. Find some kind of gov't job where he creates less value than he is getting paid but is still creating positive value
If we ignore #1 (because if you believe #1 is always the answer then the rest of the conversation is pointless) then the question is who falls into bucket 3?
My guess is that you have two types of people, people who are not capable of doing productive work (which I'd argue are very, very few) and people who will choose to be unproductive because of the jobs guarantee (they know that they have a job no matter how badly they do it).
So maybe the jobs guarantee should be combined with a UBI with differing levels of support. Provide a UBI which covers basic needs and a jobs guarantee which provides a little more money. There will always be people who want to work just for something to do and there will be people who want to earn the extra money to improve their lifestyle. Then you don't force people into jobs where they create negative value.