Growing up in that household, I have to say Taichi is much better than what he could have been.
I'm sure Taichi just didn't want Arata's involvement in their birthday party - not so much being the relay point. He doesn't like Arata and gets pissed that Chihaya is so obsessed about him.
We've seen Chihaya beat the two Rank Bs, but they've also shown that they've got plenty of skills/potential too. I want to see how long it'll take them to reach Rank A too, instead of staying in rank B (so that the writers can show that) she's that much better than them.