Same reasons as the previous. Currenty, with MySQL and EBS volumes for snapshot servers we have the relay log directory on a local ephemeral volume on slaves. If the slave falls behind (which it will during a snapshot) the relay log directory will start filling up. It would be bad for that data to hit the ebs volume as that would kill the slave. I'm assuming at least some of these issues would also affect MongoDB, considering AWS EBS performance can be flakey. This issue might also affect iSCSI volumes as well if there's a lot of load.

-Alex