the licence is GPL, LGPL, MIT, and MPL

The MIT license doesn't address the patent issue in any manner. Which is possibly one of the reasons why it's chosen. (they want Dirac to be accepted by everybody and not just the open source crowd. That means working with proprietary software and that means patent issues)

If I remember correctly the mp3 codecs from Fluendo are under the MIT license, but the patent licensing that goes along with the binaries and source code effectively make them 'proprietary' codecs.

So by releasing the codec under MIT and GPL-related licenses (which loosely covers patent-related issues in a non-bulletproof manner) Dirac is effectively assuring us that they have a commitment towards Free media codecs and keeping Dirac open that you won't get if you do pure-bsd or pure-mit license then have end users convert it over GPL.

In other words, by releasing under the GPL they are making a commitment towards covering end users from related patents they own. This commitment is not expressed if they release it under a pure-MIT license.