Fortified Superstructure greatly increases the hull and armor strength of the Ankylon Titan. It also increased the shield strength by a moderate amount. In most cases, it is a better idea to invest in Furious Defense, even when on the offensive as it actually adds more durability to the Ankylon than this upgrade.
It is still useful, if you have no other useful ability to spend a point into in your current tactical situation. Especially useful on the lower titan levels as Group Shield and Disruption Matrix is not particularly powerful there while the increase in armor of Fortified Superstructure make Furious Defense even more powerful.