Is there any benefit to using triaxial weave pattern instead of common twill?
No, it is not a gimmick. Triaxial weaving has been around for a long time, and can be seen in baskets, hats, etc. etc. It often exhibits superior structural properties such as greater resistance to shear forces, tearing, bursting etc. The sheet will also be generally more isotropic, so if you don't know which direction your forces will be coming from, it may be a better choice.
Whether there is any benefit, however, depends entirely on the specific application. You may not need any of the above properties, making the increased cost not worth it. Further, if you are exclusively loading the fibres in one orientation, then a square weave might be a better choice. What has prompted this question?