People underestimate what Pienaar does as his best work is off the ball. He's a player who knits attacks together by dropping intelligently into space, being an out ball who keeps the flow of play moving.
Say we got Atsu - you'd all notice pretty quickly that the dynamic of our left hand side would change radically, as it'd mean Baines would not advance as much as we'd be playing more direct football to get the quicker player stretching the opposition.
There's more than one way to approach the game. Pienaar offers one dynamic, whilst the likes of Deulofeu/Atsu offer an entirely different one. One style is not automatically better than the other, even if it is more eye catching.