This is a good read:
Valve's Paid 'Skyrim' Mods Are A Legal, Ethical And Creative Disaster I don't have any problem with modders receiving money for their work, but allowing mods to be paid-only like this just has too many downsides and will do more harm than good.
Where previously a mod had to be truly exceptional to go commercial, requiring special agreements to be made with all relevant rights holders (e.g.: Counter-Strike, Team Fortress, Garry's Mod, Portal, Black Mesa, DayZ) - which meant that all potential legal issues were well ironed out, there was a lot more regulation & QA over the end product and protection for buyers - now all mods no matter how trivial or poor quality can be "commercial" without much oversight or regulation, QA, etc. and little protection for buyers.
Where previously there was relatively free exchange of ideas, assets etc. between mods as long as original authors were credited, now there is a financial incentive to disallow sharing even for free mods and there is a lot of distrust, infighting and bitterness amongst the community.
These changes are fostering a lot of greed and corruption and we're already seeing "early access" mods, nagware mods, paid mods based off free mods originally made by others (which apparently is allowed) & even outright stolen mods, "shovelware" mods etc... it's all a huge mess.
In addition to the points made in the article, I fear this will also lead to yet more Steam lock-in. With "mod piracy" now a thing and the introduction of a financial incentive for publishers to lock out sites like Nexus and ModDB, I suspect it's only a matter of time until we start seeing the Steam Workshop becoming mandatory & DRM for mods being introduced (and it'll no doubt all be promoted as a positive thing for modders & the mod community, all "for their own protection").