If you only needed to replace them once, and every one then its cheaper. Especially if you add in the cost of shipping, or not riding while waiting for the right gasket.
I ordered the complete set, cause I haven't found a clear source of each gaskets shape, part number and where it goes. That's something I will do as I pull the gaskets out of the bag so that I can reorder them if I ever need them again. Or make the judgement to order the whole set again. You can always ebay the extra's.
And no, this gasket set does not include the plug.