most browsers nowadays use HTTP/1.1 and persistent connection.
Which means that 1 tcp connection is being used to send and receive multiple http requests/responses.
If you do not have persistent rebalance, the ACE module [and any other loadbalancers] will use the first request to make its loadbalancing decision and then it will leave it like this.
So, if the user clicks a link that should be sent to a different server, the ACE module will not see it.
With persistent rebalance the ACE module inspects every requests and if necessary it will remap the connection to a different server.
The only drawback is that it takes resources and times to inspect all connections. So you'll get performance not as good as if you did not have persistent rebalance.
Gilles.