Yeah, that'd work for older versions, but not for newer ones. 
Yeah, you know, the more I think about it, .. I realize what other games do. They:
1 - Require that you have DirectX9.0c or greater ( because there are things in there that the game absolutely relies upon and would be an unnecessary pain to code around )
2 - They supply the directX 9.0c installer if you dont already have it.
3 - They release 1.x patches which might take advantage of newer versions of stuff, while fixing bugs.
[rant]
Windows should basically take care of you for backwards compatibility and that's the most that you need to worry about when releasing software as a commodity. You can't really see into the future. For mouse code, DirectInput Version 8 is still the newest one. Imagine, at some point ten years down the road we'll all be using... Linux? ;-p , and people will say, "Man, I want to play Aquaria again.", and there will be a web effort to make it work on whatever the new stuff is. It doesn't need to support the newer dll, just support whichever xinput is supplied by directX9 now and that's it. Those standards wont go away for pretty much ever I think.
[/rant]