I have downloaded many of those 50 or so rar files, to create one big one. When it screwed up, I never found away to read one. It needs the information from every single one to work, you might be able to extract one, but I don't think anything would come from it.
it usually depends how they are setup. if it's split up seperate among rar files then, yes you can just extract them, however when it's split up as a whole, you won't get anything out of it usally.
Also it depends on the fileformat... WMVs and MOV files split up easily, however if it's in dvd format, for example, you won't get anything unless you download the entire set of files associated with them.
I'd check our friend google and see if you can find an app that puts together partial video codes.. (seriously doubting one exists though)