Hi,
It is at the discretion of the management and varies from one company to another. Suit yourself. If you think that the cause is genuine, you may like to consider it. But should you get a feeling that the parting employee is trying to take you for a ride, i.e., does not want to turn up during the notice period, then you may decide differently. One word of caution though, you may not wish to have someone on board who is physically present but mentally missing. They may cause more harm than help to the organization.
During the notice period, the parting employees also, at times, influence other employees. It is in the interest of the organization that they leave early and do their F&F. At times when the position is critical and a replacement is not available immediately, you may choose appropriately. :)