Concept is intrequing. modular tanks could be quite the weapon of war. switching from a cannon to a missile launcher or other weapons or utilities could be quite the asset. But I imagine due to cheaper manufacturing costs for a non modualr design, and cheaper matinence. added it probably would be quicker to build a non modualr design. However, for smaller millitary forces, specail forces, or very welthy mercenary groups might go for that tatical advantage.