OK, I'll take the consevative side.
First, human nature, if people can get money and not work they're not going to work.
Second, why should I work just to give my money to someone that wants to sit on their butt.
Third, those that did want to work, could create a huge underground economy, thereby
not contributing to the money needed for the UBI.
Fourth...
I'm a conservative (well, actually libertarian, but that makes me fiscally conservative), and I've already addressed two of these points further up thread.
First point: I disagree with your assessment of human nature. Most people want to live a meaningful life, however they define that. It's probably true that the TYPE of work that many people would choose to do if they didn't have to work just to be able to survive would be different than what they currently do, but I don't view that as being necessarily a bad thing. If someone wishes to be an artist, or write a novel, or whatever else, they will have that ability. You should consider too that many people do a great deal of unpaid work that society is absolutely dependent upon, such as caring for children or elderly relatives, volunteering for charity, etc. So this would free them up to be able to continue to do such necessary but unpaid work, or to otherwise pursue whatever they deem to be of greatest value to them as human beings. UBI would also greatly decrease government control and interference in people's personal lives, since recipients, and not the government, would be the ones deciding how to use the money. It would ideally lead to smaller government and less bureaucracy, which are ostensibly conservative goals.
Second point: you're already doing that anyway, assuming you aren't evading your taxes. If you're a conservative, then I doubt I need to point out to you how profligately wasteful our current welfare system is. In fact, by reducing people's welfare benefits if they take low wage jobs, we are currently literally paying people not to work. We also pay farmers not to grow crops and all kinds of other corporate welfare stupidity that would ideally go away if UBI were adopted.
Third point: again, this already occurs to some extent under the current system. It's not clear to me why this would be worse under a UBI system than it currently is.
To me, the potential of UBI to maximize personal freedom while minimizing government control over people's personal lives and choices is its greatest appeal. However, there is also the added benefit that it would provide for some income for people who can't find work in the future, given that many currently undesirable and low wage jobs may not even be available in the future. When the cars start driving themselves, the planes are flying themselves, the buildings self-clean, and we order almost everything we need or want online, what kind of low wage jobs do you anticipate still being available? And what would you propose doing instead with the "surplus" workers who can't find paid work?