I said nothing about incompatibility with other licenses. I said that by licensing your code under MIT, you're restricted from using L/GPL code, while the opposite is not true, and if you're going to use code that imposes restrictions anyway (like MIT-licensed code), you're better off allowing yourself to use GPL-licensed code as well.
The MIT license does not prohibit using GPL, unless there is a license incompatibility (which I do not see). You can of course prohibit yourself but that has nothing to do with the license. If you release that code, other people can still combine with whatever regardless of what you restrict yourself to.
The GPL on the other hand does prohibit many licenses. Whether you want it or not. Or whether the people who use your code want it or not.
A license that you choose personally never prohibits you from doing anything, because you're the copyright holder, so you don't need a license at all. So choosing the GPL for your doesn't prohibit you from doing anything either. What it does is allow you to use someone else's GPL code, which the MIT doesn't.
Indeed. In fact since the MIT license explicitly grants sublicensing, you can take MIT-licensed software and simply offer it with no modifications under the GPL. This is not safe to do with the BSD license btw.
