The Beethoven's fifth you linked to is performed by a small town college orchestra, not the Musopen Symphony Orchestra (really the Czech National Symphony Orchestra, but I don't know if Musopen are allowed to say that in advertising!). Anyone can contribute to Musopen - you'll even find midi keyboard renditions there. It's better than nothing, is Musopen's philosophy.
You still have the option to pay money to hear Bernstein's interpretations. In fact, you probably will always have to pay money to hear Bernstein's interpretations, the way copyrights are being extended... but now you also have the option of hearing some solid renditions of Brahms symphonies by a professional Czech orchestra, for free. For ever.
I think the project is valuable, I like the idea of having good recordings of these works available for free. For those who can't afford to spend the money or who want to reuse the works in some creative way, this is a real boon. It opens up the music to a lot of people who wouldn't otherwise have access to it.
On the other hand, I don't think the project is going to have any real impact (at least not yet) on the community of listeners who have a love of the music and the resources to indulge their passion. I buy lots of music and will continue to buy lots of music. Not because I enjoy spending the money, but because it's more important to me to have a recording (or even multiple recordings) that I really like. The cost relative to my enjoyment is really pretty minimal. If the free recording is worth having, I'll add it to my library. But I'm happy to pay for a recording if I like that one better.
And I'm not real hopeful that these recordings are quite there yet. I found the Musopen Symphony Orchestra recording of Beethoven's Symphony No. 3. My initial take was that the playing was good, but the sound quality was less than ideal. For example, there's an intermittant but very distracting hissing noise at the beginning. I'm not an audio technician and I couldn't tell you what causes it, but I know that it makes the recording less desirable for me.
Still, I'm glad the project exists. Making this music available to the widest possible audience is a good thing. That's something to applaud.