A distinction needs to be made or clarified here. When people talk about lovers being their best friend they're not necessarily talking about being friends or having a relationship with their lover like they do with their good friends. They're not talking about having a lover who is going to be their wing man / woman, invite them to house parties or any of that stuff. UNLESS, they are... then please correct me.
The term best friend here refers to a relationship that is not just passionately driven, but also founded on platonic friendship-- in other words, a profound understanding of one another. But such relationships takes time to form, and ultimately really depends on the compatibility of the couple, in which case, it may or may never happen for some.
These kind of relationships should be pursued but they should never be forced.