This argument is moot when you're streaming some blockbuster movie, there will not be any rare chunks to speak of. The client could implement a threshold where sequential downloading is only switched on when there are x number of seeders.
I agree in spirit, but the idea of everyone starting at the same end of the download does indeed go against some of the core ideas of bit torrent.
Personally I think maybe something should be built on top of bit torrent to accomplish this task. To me it's a case of being careful when re-engineering underlying protocols because of a higher level need.
Anyway, it's a nice hack, but I kind of understand where the client developers who refuse to do this are coming from :-)
