While I would certainly prefer to see the major players leading the way in adoption of AES by default, RC4-MD5 persists for at least 2 reasons:

1) Habit

2) As implemented/deployed in SSL, it still provides some security

RC4 has gotten a bad reputation in large part because of its poor application in WEP that resulted in keys being rapidly recovered by sniffing traffic. The Wikipedia entry is a good place to start http://en.wikipedia.org/wiki/RC4#Security (& numerous references for the original papers/pubs cracking various bits of RC4). The RSA response to RC4 concerns (from WEP) is worth reading, as well http://www.rsa.com/rsalabs/node.asp?id=2009 .