I see this as a neutral/positive change, probably because I don't use any addons that require payment and also because I'm only an end user of software development. It could be seen as a move to nip this sort of behaviour in the bud, I don't know how much Carbonite costs, but there is a risk that other addon developers would see payment as a good option. There's not many people who wouldn't like a bit of extra income and for people making good quality addons and keeping them updated regularly for fun it could be very appealing to charge for it too if it seems that the market is out there. If it did become a widespread thing though it creates a divide in game experience between those who are willing/able to pay for addons versus those who are not. There are plenty of addons which aren't really important for game play, but there are many that do provide an edge in various areas (e.g. DBM, omen, healbot).

As a slight aside, in the comments on the WoWInsider article someone was expressing concern that Blizzard could use this to put a stop to addons such as decursive or cooldown timers. As a raider my first reaction was that this would be a terrible thing, on the flip side though many people are complaining about how easy the game is at the moment. This is partly to do with encounter design, but it can't help that for most things it's much easier to get an addon than actually pay attention to what's going on in the game. I'd hate to have to use the default action bars and unit frames, but I can't say I'd be that dissapointed if DBM was removed and you had to pay attention to casting animations on bosses rather than a warning bar on your screen (though I can imagine it would make things a lot harder for guild, like mine, who have a fair few people who can't manage even with addons).