Whether you choose to add Yakima Same Key System lock cores or not really depends on exposure to risk. Expensive gear? Leave gear unattended? Unfamiliar environment? Strangers in the area? These are some factors to consider when deciding if locks would be worthwhile.
Yakima designs strong theft deterrents into their products. Some products have the lock cores included others require the lock cores to be purchased separately. As you add carriers, you can use extra cores from your pack fit all your Yakima Same Key System "SKS" products with the same key. Brilliant!