The big reason that sharing old versions of the Minecraft Client is illegal is because it counts as redistribution of Notch's code. It is however possible to downgrade without needing a old .jar file.
The linked tool allows you to download patch files to revert to older versions. The reason why this is legal is because it is modifying the code rather than redistributing original code. We know that Notch has no problems with people modifying the code otherwise our very own bukkit powered server would be illegal!
As linked below by Darren here is a better tool for downgrading minecraft which seems to be better supported by the developer and works for Mac, Linux and Windows
See you in blockspace
Edited by Grindboy, 30 May 2011 - 05:51 PM.