Who said they wanted to heal something else? Apart from the apparently iffy targeting, the main reasoning against mo heals is that you can't queue them, costing you time between heals, and to minimize that time loss you have to trigger the button to lose as little time as possible, which sounds extremely annoying.

But there even is a drawback concerning the mouse cursor: With mo, you have to be with the mouse over your target the moment you want to start casting your heal spell. If you work with targetting, you just have to select the target at some point in the 2 to 2.5 seconds before you have to cast the heal, so if you select early in that time frame for one spell, and late for the next spell, you have about 4 seconds to do something with your mouse. For example you could get a better overview about the battlefield by panning the camera around or something like that. You are simply more flexible with what to do with your mouse, even though in most cases it won't matter.