If you have the technology to build a layered defense you should do it and it is effective.
You need to consider the fact that using a variety of weapons means you spread out the cost of your ships over a wider variety of materials which in effect make your fleet cheaper since each mine produce some of every resource on a planet.
If you elect to concentrate on one weapon system you will quickly find out how those minerals become scarce when maintenance, building and upgrading fleets are based on a few mineral types.
So, in my opinion, efficiency is more than just weapon stats... it is more about your overall economy than anything else.
Using a layered defense with Gauss, Lasers and Missiles for defense is for the most part an effective combination.
You obviously need turreted weapons to effectively engage missiles. The math for none turreted weapons such as Railguns are only half effective at low technology levels.