There is a couple of issues with TLS/SSL renegotiation vulnerability in the context of HTTPS protocol, which appear not to have made their way to the public.
1. Plain text prefix injection is not the only risk. The original advisory  mentions the possibility of "forwarding and repurposing of client certificate authentication credentials". In oss-sec maillist Marsh Ray goes in more details , and  dedicates one slide to "client certificate redirection".
2. The renegotiation vulnerability provides for an additional attack vector to exploit web application vulnerabilities. For example, MiTM attackers can use it to deliver an exploit for a non-persistent XSS bug to client's browser.