It's literally some 40 lines of lua code in a WoW addon, albeit with necessary tie-ins to a prior UI skinning addon (some 13 assets and another 100 lines, for less than a megabyte of space and memory cost barely enough to be registered in the few events that it's running). Granted, it may well take years for SE to do what a casual programmer did in a few hours there. But my point was that simply "bloat" is rarely ever the issue.
A gauge mechanic would obviously work, too, but requires that the whole job be built around multiple elements of regular attacks just to allow for a difference in pets and thereby additional summoned capabilities (through said pet). You can abstract the controls, saving space, or you can couple it to another, again saving space, but also reducing free control. Those restrictions might increase the interest of gameplay, or they might not; it all depends on how it fits together. But facing bloat as a matter simply of control systems, rather than by surrendering that control to yet another system, solves it then and there.
