Confused about persistent connection for old clients

OK, I too, am confused about why proxies MUST NOT establish
persistent connections with 1.0 clients.  If the client and
origin server connections are handled separately, and if the
proxy understands the 1.0 Keep-alive, what's the danger?

> 8.1.3	Proxy Servers
>
> It is especially important that proxies correctly implement 
> the properties of the Connection header field as specified in 14.2.1.
> The proxy server MUST signal persistent connections 
> separately with its clients and the origin servers (or other 
> proxy servers) that it connects to. Each persistent 
> connection applies to only one transport link.
>
> A proxy server MUST NOT establish a persistent connection 
> with an HTTP/1.0 client (but see RFC 2068 for information 
> about the Keep-Alive header implemented by many HTTP/1.0 clients).