2004 Subaru Forester will tail light crack do damage

I have a 2004 Subaru Forester XT. Somebody hit my bumper and cracked my tailight when I wasn't around to see it happen. When I took it to the autobody shop to get the bumper looked at, the man working there told me that I should also get a new tailight because water will work its way in there and cause damage. It is a fairly small crack that lies mostly along the bottom of the tail light. Do you think this guys is pulling my leg, or do you think I should order and install a new tail light??