I think that there is use for it. It’s pretty clear that the seme isn’t into the sex as more than an act to relieve himself and exact revenge. He is using sex as a tool. I think that when the seme and uke finally have sex, there will be a stark contrast where he isn’t using sex, but rather having sex where he is concerned for his partner, rather than his plots or ploys. The straight sex scene sets up his character and the possible change in his character. It is a rather succinct way of communicating the possible changes and setting up the drama and cognitive dissonance he will inevitably feel when his revenge takes a back seat for perhaps the first time in his life. Just my two cents, though.
And it’s mobile, so he can follow Wataru-san anywhere. I’m literally crying.