Tanabe is not a complete waste of ink. He serves as a yardstick, a measurement. Compare Tomo's progress with Jun since chapter one, while Tanabe has made zero progress. Compare how Jun's feelings have changed regarding Tomo, while Tanabe is still beneath Misuzu's notice. - Granted, that's not saying much, Misuzu only cares about Tomo and Carol. His only advantage is how he's not being actively despised by Misuzu like how Jun is, and not being plotted against by her.