The crack, as you know, is where the fiberglass meets the lead or cast iron, which are two different materials. Bad scenarios are loose keel bolts. If the keel bolts are tight and no water is coming into the bilge from below, then you don't have a lot of concerns. When the manufacture puts on a keel, often the shape of the keel does not match up perfectly with the shape of the hull (usually a shoe which also forms the bilge). So, an epoxy fairing compound is trawled in to smoothen the joint up. If there is fiberglass added on top of that, then that give the joint a finish that will not produce a crack. If only fairing compound is used, then a crack often will appear because of the minute movement of the two materials. The strength of the keel to the hull is in the bolting process and not by fiberglassing so much on the outside. In other words, if the keel bolts are not tight and secure, the fiberglass on the outside cannot be expected to do the work alone. Walk about a boat yard and you will see many boats with this smile. It is not something to loose sleep over unless there is another problem.