The logic that because you have a significant other, you don't need friends is mind boggling. People often say, “My other half is my best friend.” A best friend means the cream of the crop. You can confide anything in them without fear or judgment. If your boyfriend/girlfriend/husband/wife isn't one of your best friends, you may have a whole other problem with your relationship.
If you're having a bad day people will often say, “where's your boyfriend?” Why yes, he's right here I carry him in pocket at all times to ensure he will always be there for me. It's great to have an all in one doll to take out with you, romance, and watch the game with; but the truth is everyone needs friends. Even if you only have one good friend outside of your significant other, to have a well rounded life one person cannot be your entire world.
“But I love them so much!” you cry. It doesn't make you love them any less because you can love other people too. And for the friends that pass off their friends to their significant others, shame on you! Your friend may be looking for outside support! True friend stand by you no matter who you're dating. Significant others stand by you no matter who your friends are.
The exception to the rule is when one of those relationships is harming you in some way. That's when the people who care step in. Maybe the solution is that we don't need just one person who is our go to for everything; maybe having a circle of caring go-to people is more satisfying.