The most basic function for BOFFs is to provide space and ground abilities. From this "BOFFs=gear" point of view, you really don't need that many to play the game effectively.
However, many of us play MMOs for more than just gear/stats requirements, and I think BOFFs are a great way to add a personal touch to one's crew that adds to the game's enjoyment immensely. I have 29 BOFFs - way more than technically needed - but I think it's fun to customize a crew with a diversity of races. I think it's great to have a different set of faces every time I beam down, and to see so many of them on my bridge. I even have them spread out from ensign to commander so I don't have two dozen commanders on my ship or away teams.
Who else takes BOFFs past the "gear" level and makes them part of your personal STO story?