Good question. It's obviously a complicated affair, one person's sugar daddy may simply be generous or have no intent of being a sugar daddy.
My knee jerk is that they appear to me to be exploitative and I don't doubt some are, going both ways. I don't see it beyond the realms of possibility that an infatuated and lonely person is used in much the same way as someone may befriend an old person to get a chunk of a will. The receiver may also be 'sucked in' and feel valued but be very mistaken about the manner of that value. In this I include certain relationships that involve 'tribute', they don't sit easy with me for the reason above. I can't help but feel it preys on a vulnerable element within someone, I hate to say that because I know I know very little of the issue in that way and many tribute givers likely feel empowered but given my own experience there appears to be something 'unnatural' in 'paying' for certain experiences that involve such complicated psychology and that should be free in conventional relationships. I'm aware not all will agree of like this view and I don't myself, it's not very sex positive but something just doesn't sit right for me. I really hop that doesn't offend and I would love to hear the other side and clarification.
I know not all relationships of this nature will be exploitative, it may just be another dimension to the sex trade that suits both people (I think it is part of the sex trade because if the answer to the question 'Would you if you weren't rewarded?' is no then it's a transaction, albeit a non-explicit one). If it does suit both then it's an elegant solution to a problem. I don't think it is inherently unhealthy, if both parties are strong and aware of their boundaries but I still think the potential for abuse makes it an uncomfortable idea to me.
So, if both parties can be empowered by it (like any sexual practice, oral sex, swinging, voyeurism etc) then it's a great thing, I just think it requires certain types of people or it can go rather tits up.
There are of course wide social implications to contend with, although two parties may feel it suits them perfectly it takes alot of strength to breach certain social barriers (such as age or it being seen as exploitative). That is another dimension to address.