I would never use a B-52 to launch a harpoon when an S-3 was available, do you think its ever been done?
I don't think either has been done in anger - could be wrong.
The idea of the B-52 harpoon strike in the 80's/90's was mass. A Sqn of 8 BUFFs with 12 Harpoon each would add a lot of firepower to a CVN's ALPHA Strike on a Soviet SAG or Carrier Group.
I believe the doctrine was that the CVW would 'break-in' to the defences and do as much SEAD of the Ships defences as it could, the A-6/F-18 strike would work on the escorts and the BUFFs would come in shortly afterwards with for the HVUs.
I think the S-3 (and P-3 for that matter) use for the Harpoon was more in the armed Maritime Patrol role where you are so far from quick support you want to be armed with both ASW and ASuW munitions to tackle any target of opportunity.
B