I work at one of the mammoth consulting firms doing Salesforce implementation work for Fortune 50 enterprises, and you would be amazed at the amount of client value derived (and the # of licenses sold) utilizing the AppExchange
I do know that the value can be huge- we are using a number of them in our org too. Heck,I even wrote some smaller Lighting components myself( working on another as I type), which boosted productivity quite a lot.