its definitely not pure bronze but a mixture of alloys.
anyway I think the draw of it is over time it does changes colour and THAT gives it the added character that some collectors look out for. Personally I think it's cool and love to have one.
Apart from the big boys as mentioned the micro watch brands like helson are doing in their shark diver range. I much prefer the Benarus moray with a brown dial.
(image shamelessly taken from benarus blog)here you can see how the colours have changed.
In hindsight, Anonimo actually did launch a limited edition Millemetri in bronze for the JP market back in 2007. It's just a case of the 'fad' catching on with the other brands I suppose....