This statement always make me confuse. Because I often hear that the Enterprise D is an exploration ship. But despite of that, many episodes that I watched long time ago suggest that this ship wasn't truly do explore. They tend to patrol the Romulan border, Defending Earth from Borg, visiting the Federation members, hunting Marquis Rebel, etc. So how can an exploration ship travel inside the Federation Space more than going into the outer Rim, and stay there for years in an exploration purpose? I think The USS Enterprise D mission was not truly to explore the unknown. Their mission was to expand the presence of the Federation in their immediate sector. Or more precisely, act as the Federation representative in their mission area. That's why when the Borg came in the final episode of season 3, they came after Picard. Because Picard knows about all Federation military element in the sector. He's more than a captain of a starship. He should be an admiral, because his duty was more of an admiral duty than a starship captain.