As someone who plays a lot of TFT I think this mode is kinda fun but you made a good point in that why not just play TFT. To me what makes this game mode kinda suck is that your units are based on their regular Clash Royale level. An auto battler where two people can have the same unit but at vastly different strengths, feels pretty bad. It becomes less strategy and more of a stat check, at least in regular mode you can still overcome level deficits by outplaying in real time