What we need is to bring performance to the point where it does not matter any more, so all the inherent advantages of OSS drivers: KMS, out-of-the-box support, integration, support for X technologies, lack of spyware and malware, longer support, etc. take over.
With r300g, we're already there. With r600g, it will take a while longer, but we'll get to 65-75% of the performance, and that's fast enough for most people. There are always people who will fuck around with blobs, dicking them into a kernel that was not designed for them, to obtain 30% more FPS, but I imagine that for most users, this perversion will become unnecessary, just like nobody is installing nforce ethernet binary driver nowadays.
It's like GCC vs. ICC. ICC is much faster, but everyone uses GCC anyway, because speed is not everything. We need to cover the needs of the computer users, free software itself is THE killer argument.