Good question. It seems very random. Two people upload the same video. One gets taken down, the other doesn't. Their songs are still up there; it's just now there are only 5 versions of the song instead of 10. Thus, they haven't even accomplished their most basic goal of protecting their copyright, since the material remains. Putting aside the damage to public relations, the YouTube purges are failures in practical terms as well.
Also, something I mentioned earlier that I find very ironic is that the uploading of commercially available material such as songs from HFO and F1 seems to be largely left alone, while the fanvids and fan covers are targeted by whoever is making the complaints on behalf of Henley. (For instance, you can easily find The Long Run from Farewell 1 currently on YouTube - here's an example.)
That is inexplicable to me. It's like they are TRYING to alienate fans!