[Juliana Maria]

Some just friend you for the benefit..

For example, if you have a war galleon and they don't.. they will use you for your ship..
If your on an island they need and they don't have tp access to, they will use you as a tp totem..

And some, as Maria said.. just care about the numbers...

In the end, just stick to befriending the pirates you've spent sometime with and have gotten to know.. not the run-by invites...