Split Second or Can not Be Countered
Split Second has uses other than making a spell uncounterable.

As a simple example, say you cast Sudden Shock; targeting a creature with only 2 toughness. If not for Split Second, your opponent could save the creature by casting a spell any spell that does any of the following: 


Increases the creature’s toughness 
Gives the creature Hexproof
Gives the creature protection from your spell
Regenerates the creature
Prevents the damage


Several ways he could save his creature. But due to Split Second, he can’t respond at all; and the creature will almost certainly die. 

Similarly if you cast Sudden Shock targeting your opponent who has 2 life. He can’t cast anything that will increase his own life. 

There are all sorts of ways other than countering a spell that you might want to respond to a spell. Split Second prevents all of them.

While Split Second is generally more powerful than “cannot be countered” due to the reasons stated; in some cases “cannot be countered” will work better, such as if there’s a triggered ability that counters spells, such as the emblem from Jace, Unraveler of Secrets.