Basically, Matthew explains how Canonical is trying to push Ubuntu (in the form of Ubuntu Phone/Touch) into markets generally hostile towards the GPLv3 licensem since the license requires users be able to replace the GPLv3 code. Android and other open-source mobile platforms tend to be under a more liberal license that keeps open-source enthusiasts happy along with mobile phone vendors.
With Canonical being the sole copyright holder to Mir (and other key components), due to contributors signing the CLA, they are able to exclusively relicense the code-base to their hardware partners and meet their needs. However, this isn't exactly good for free software advocates.
Matthew concludes in his Canonical CLA blog post, "If Canonical were releasing software under GPLv3 because of a commitment to free software then that would be an amazing thing. But it's pretty much impossible to square the CLA's requirement that contributors grant Canonical the right to ship under a proprietary license with a commitment to free software. Instead you end up with a situation that looks awfully like Canonical wanting to squash competition by making it impossible for anyone else to sell modified versions of Canonical's software in the same market. Canonical aren't doing anything illegal or immoral here. They're free to run their projects in any way they choose. But retaining the right to produce proprietary versions of external contributions without granting equivalent reciprocal rights isn't consistent with caring about free software or contributing to the wider Linux community, especially if it means you get to exclude those external contributors from the market you're selling their code into."