Hi,
With igmp-snooping the switch is only examining IGMP messages to build a multicast cache table and not really interfering with the IGMP messages. If an IGMP leave report is sent by one of the servers, this will be relayed to the upstream ae0 multicast-router interface, which I believe is the desired behavior.
The eventual outcome depends on the design/topology whether there's a multicast router [IGMP querier] in the path towards multicast-router interface. If a multicast router is present, it will send back group-specific-query in response to a host leaving the multicast group and allows the router to quickly determine if any remaining hosts are interested in the group.
Below link provides a nice illustration of the scenarios:
http://www.juniper.net/techpubs/en_US/junos13.2/topics/concept/igmp-snooping-ex-series-overview.html
Could you share your topology. I believe the eventual design & config would depend on the topology.
__________________________________________________________________________________________
even more, enabling IGMP snooping on the Juniper could probably imply performance impact if it must "read" all the IGMP traffics sent to a Mcast group address (used on service traffic as well as IGMP memership reports)
__________________________________________________________________________________________
I guess this is a tradeoff between switch performance and multicast flooding. The benefits of IGMP snooping would most probably outweigh the performance drawbacks.
Cheers,
Ashvin