Well, for the format based ones, it truly is a case of "if you don't know, you don't need to know". Mrsid for example is a rare format that you are extremely unlikely to ever encounter, thus you are very unlikely to ever need to open it.
Function based addons though are an entirely different story. Harry's filters for example are a bunch of filters to tinker with your images (it's a large range of filters, from color effect to tiled effects to blurs, etc, etc), and even if you had never heard of it before you might well find it useful (or at least fun to toy around with).
Does the installer or anything at least indicate which ones are format based plugins vs which ones are for effects or functions? (I never bother with the "extended" version of xnview, nor with installers. I always grab the minimal version & the full plugins pack, zip version in both cases.) Format vs function is important to know.
In the meantime, there is a place where you can find a bit about all the addons on the site, though it's kinda buried with the new layout, here's a direct link:
http://www.xnview.com/en/xnview/#addons
There are aboput a dozen addons which provide extended functionality, the rest are all for various image formats.
