Small little DLCs like Horse Armor should be free, and most of the hate I see toward DLC is aimed at stuff like that costing money. Real expansion packs or "mini-expansions" like Fallout 3's DLC or Mass Effect 2's Shadowbroker and Overlord missions can and should cost money though, and are worth it usually.
Hmmm, what I have noticed is that DLC smaller than what used to be expansions is usually low quality or lower quality than what expansions used to be. For example, I heard that the expansions for Fallout 3 are not very good, while people are much more satisfied with TES expansions. If, instead of making 5 little expansions they focus on making two considerable ones, the quality may have been better.
What I mean is that any extra content is typically of lower quality and is worse/more trivial the smaller it is. There seems to be something in the development time of those (maybe shorter times, more burnt developers, the need to create something while the release is still hot...) that results in this. I wont say they should or shouldn't charge for mid-size extra content (the ones you mention as mini-expansions), but anything that is smaller than traditional expansion packs will make me respect the developer a little bit less.