Although I think people should be rewarded for their work I don't really don't like where this is headed. As others have noted the statement from Valve reads as if paid mods are definitely coming in a not so distant future. Modders getting paid is all well and dandy until you consider the way mods often evolve and build on top of each other.
I spent a lot of my time as a young teenager on the custom ladder in Warcraft 3, where people played home made maps which essentially amounted to mods. This was the birthplace of Dota and the place where many of us were introduced to tower defence. Every time someone uploaded an original or fun new map people would take their map and tweak it in some fashion or another and slowly the maps would evolve. The versions of Wintermaul that people were playing years after the original were definitely improvements upon the original.
If new maps/mods on the custom ladder had cost money this would never had happened. I'm sure Duke Wintermaul wouldn't have been happy with all the remixes of his vastly popular mod if he had been selling it himself. And, although he might have updated his mod to improve balance and the such he most likely would not have come up with several of the features that were included in the late versions made by others.
I see people saying that modding is for hobbyists and I have seen several modders claiming that they will never charge for a mod and that may very well be the case. For now. Once paid mods are released they will slowly seep into the community and the modders of the future will have been raised with paid mods instead of free ones. Once upon a time video game companies would sell their games in retail and then support it for free, now they charge for the support cost through subscription fees, dlc or microtransactions. If customers are OK with paying for it, why should they give away their work for free? The same thing goes for modders.