The best use of turtles in base defense (especially on a primitive server) is a whole group of them. When you get raided they will swarms the gigas and rexes rendering immobile. That allows your plant X and turrets to go to work for longer. And nothing can tank like a well leveled turtle with its shell defense and a (hopefully bumped up) saddle
First dino I ever tamed and bred. In my opinion its a slow breeder. But hell of a defender. Its just a good tame all around especially due to the fact its 10x better then a trike (in defense) and the saddle is relatively easy to unlock. Plus its got a cool swimming animation.