According to the book, Data Center Fundamentals, page 369, SSLv3 support was added in Netscape 2.x and Internet Explorer 3.x, and TLS was added in Netscape 4.x and Internet Explorer 4.x.
I am listening router in my company(I have 200mb pcap file with just https connection) and these old clients does not exist in company computers but I observe some of the client hello requests are in SSLv2 format. So only reason a client send client hello in SSLv2 format seems compatibility with servers who does not support new versions of ssl . This compatibility issue stated in http://tools.ietf.org/html/rfc5246#appendix-E.2 .
I couldn't understand when client send "client hello" in v2 for compatibility . How client decides it should send v2 client hello or how it decides it should send v3.1 for example ?