Nets declared within a simple interface is inout by default and hence any module connected to the same net, can either drive values or take values from it. In simple words, there are no restrictions on direction of value propagation. You could end up with an X on the net because both the testbench and the design are driving two different values to the same interface net. Special care should be taken by the testbench writer to ensure that such a situation does not happen.
This can be inherently avoided by the use of modports. Modport lists with directions are defined in an interface to impose certain restrictions on interface access within a module. The keyword modport indicates that the directions are declared as if inside the module.