That would be like asking "Should we pay for art that these people make for us?" Yes, some people sell their skins and others do not. I am a free skin maker for people, but I believe if artists are taking their time to make and customize your own personal avatar on MC. Then yes, it is very fair to at least get some reward in it for their effort because they devoted some of their time to make something for a stranger.
Do you think we should pay for skins when people can charge nothing?!?