If you meant stickyness instead of persistence, the reason is that the info that is passed is a cookie.

A cookie is useful because each client receiving a cookie will save it and reuse it each time it connects to the same website.

So, this guarantees the ACE module that looking at the cookie we can recognize the client and select the appropriate server.

If you prefer to use another method for sitckyness like sticky srcip, the problem is the mega proxies - tons of client sitting behind 1 device that makes all the requests.

Gilles.