They're too confusing. You should immediately be able to tell when units are destroyed. Yet it takes some units 2 full seconds or so to collapse.
It's not realistic. I mean, you get a Zergling to impale a Ghost several times, and as soon as its HP is 0, the Zergling just leaves the Ghost and moves on, as if it knows exactly at what point that happens. And it hinders the game too since it's graphically hard to tell what units are alive and what aren't.
For me and for many other ppl, SC is the ultimate STRATEGY game.However I think we should not neglect other features that can make this game even better.One of these is the "coolness factor" (since it is not disturbing too much other factors like realism and strategy.)
Now, regarding the confusion you mentioned, my experience says that after a few battles, we will be able to easily recognize active from inactive units.