Eloqua: Why have the Campaign Re-entry feature when it DOES NOT WORK?

In Eloqua, as one creates a campaign, within the campaign settings, there is a feature that is unchecked: "Allow contacts to enter the campaign more than once."

When I select this feature to be implemented, I get an error every time:

I've searched high and low on all forums and community portals and there is very insufficient information on the logic behind this feature and the error it keeps giving me.

I am aware that is this a fail-safe setting, but if this is the case, why have this feature showing in the campaign settings if it can never be used?