That is the proof that managing simutlaneously army and spellcasters is not that easy as some clowns try to make it.
A unit that requires full-attention, positioning, firing, guiding the Nova (optional but rewarding) all while Sentries, Templars, Stalkers and the bulk of the army requires attention is not that easy after all…
If a Protoss has Disruptors not in a separate Control-Group (that follows Immortals or Carriers), then they deserve to loose and you see Disruptors moving stupidly like Zealots charging forward.