Current;y, mongodb repairs databases by creating a temporary directory within it's database path directory. This is highly bad for many reasons:

1) On AWS with EBS volumes this will double or triple repair time, and cost more money if a lot of data is to be repaired. Also, by 'activating' more ebs blocks then will actually be used, snapshots will double in size and costs will double here as well.
2) For performance, the temp directory might be on a separate volume, ram disk, etc.
3) The data directory might be on iscsi (ala SoftLayer) and performance and space consideration would be an issue.
4) There might simply not be double the database space on the volume.

Relocating the temp directory and making the variable user configurable is the sane thing to do here.

-Alex