My understanding of permissive licenses (MIT, BSD) is that they allow a user to re-license the software, provided that they attribute moderate portions to the original author. Is there a reason why anybody would dual-license with something like MIT and GPL, if the MIT license would allow any person to take the code and license it as GPL? I know JQuery does this, but I don't understand why. Am I misunderstanding the MIT license or is it just redundant?
Even if technically a permissive license is compatible with the GPL, sometimes explicitness pays. Do you really want some company with uber-anal lawyers to prevent its employees from using your permissively licensed code on the one in a million chance that some tiny detail of the license you chose makes it technically incompatible with the GPL (even though that's not what you intended)? If not, then unless your permissive license is very widely considered compatible with the GPL, you should dual license it.
It is most likely just redundant. There are several versions of the MIT license, and some of them were incompatible with the GPL v2 due to the attribution clause in the MIT license. Licensing under several popular licenses allows the code to be used without having to worry about licensing compatibilities.
Sometimes it's a case where they changed to a more permissive license, and they keep the old license around just for people who are already using it under the old license. It can be a big deal for people to switch.
For instance Qt added LGPL (v2) but still has GPL (v3). They do say that you must choose a license when you begin. You can't choose one and then switch later. I'm not sure why that is...