I think the ties are in the game to fill the path-finder role mostly. Well, defenders are working great, and if you scout with phantom you can retreat with no loss.
Interdictor sucks as we all know it. They have one big use though. When you have already won the game, and that annoying annoying enemy are popping around your planets prolonging his death, its time to build that Interdictor or two to finish them once and for all. But yeah I wish it has a more effective disruption against incoming missiles and a more reasonable gun, then it will be more useable than something other than a clean up unit of a game already won.