At the least, it should allow import of data from other providers ( turbotax, etc ) so that data can be stored for as long as one likes.
A big advantage is to allow amended returns ( example I need to amend my 2009 return ) - One just pulls out the 2009 code base, put in the previous data from turbo tax and amend it.
from an end user perspective, if it's already free then being OS doesn't really add any value
Off the top of my head... Extendability, extra features and integrations, community updates, etc... __It depends on who the end user is__
If you're referring to the common user, probably not. But I don't believe thats what OP was suggesting. I'd be within the target audience who would want an open source version.