This has no effect.
What is no effect? Do you mean that the compaction fails to start? or fails to complete? or that it completes successfully, but your (unspecified) problem persists?
It is quite contemptible that Microsoft does not supply a simple option, or an external utility, for rebuilding the index, which corrupts so easily.
The database used by Windows Live Mail is a very robust one, capable of recovering from most minor corruptions with no user intervention. That's why the compaction (re-indexation) is set by default only to run once every 100 program starts.
If you're finding that the index corrupts so easily, you should be trying to find out why this is happening, because it's not normal. The usual cause when it does happen is not allowing the program to complete its indexation in peace.