chaining and proxy

Hello.

I successfully setup the chain overlay, so as to push changes from a 
slave to a master, with something as:
overlay             chain
chain-uri           "ldap://ldap1.domain.tld"
chain-idassert-bind bindmethod="simple"
                     binddn="cn=chain,ou=roles,dc=domain,dc=tld"
                     credentials="s3cr3t"
                     mode="self"
chain-idassert-authzFrom "*"
chain-tls           start
chain-return-error  TRUE

I'm curious, tough, why the slave has to use a proxy identity to 
authenticate on the master, instead of reusing original query 
credentials. Is there something preventing it, or is just that all 
examples I found sofar were using it ?

I was also curious to know if the slapauth tool was usable to test such 
kind of proxy setup. Reading the man page, it seems rather adapted to 
testing identity mapping through authz-regexp directives.