Many FLOSS projects use CLAs, including Qt, OpenOffice.org and most of the GNU toolchain, and the danger of closing these is rather small.
The only question is whether people trust the organisation to which they transfer their copyright. In the case of GNU, certainly. In the case of Trolltech/Digia, sure (the closed version funds the development and there are contractual safeguards to keep the code free forever). In the case of Oracle, not so much (hence the LibreOffice fork).
So far, Canonical has mostly released FLOSS software. The fear of CLA is a measure of distrust of Canonical as an entity. The Free Software Foundation could close GCC and make it proprietary at any point
If you want to commit to it you need to sign this CLA. That's why this project will never be "free" and should be abandoned asap. Everyone supporting this project is just feeding Canonical with his hard work with the risk of it becoming proprietary and "intellectual property".