I understand that the backseat position felt cool and it felt like you could influence some stuff, but it was ultimately completely useless as a Mario Kart mode, man. It's just not fun to play that way. If you were constantly doing stuff, then yes. But any game that has you wait for anything to happen while the other player plays happily while you do nothing most of the time, is just not a good one.
Thing is, I never really viewed it that way. I personally enjoy doing tactical stuff, so the backseat position kind of fit me like a glove. Really, in the crazy world of Mario Kart you're never really sitting there and doing nothing. You're either watching for karts to punch/steal from, being aware of collected items, and watching the road so that you can correct a mistake if/when the driver makes one. "Constantly doing stuff" doesn't exactly have to constitute of something that's more 'action-packed', as the backseat driver carries an equal importance to the driver, it's just a different style of play altogether. Quick reaction and a sharp eye both go along way in either position, and one can't really function without the other. I personally had fun being the backseat driver, and certainly wouldn't call it a useless mode. If it ever comes back it'd be neat as an optional mode or something. More options are never a bad thing when it comes to games, I'd say.
And I liked that gimmick, but I thought Rainbow Road in Super Circuit was needlessly hard compared to other games, not to mention a bit underwhelming when compared to 64's version. By the way, I just played the game just now. Just for curiosity. And amazingly... I think it feels like the most responsive of Mario Karts to me. It's probably because I spent so many years with it, but I haven't played it in a long, looooong time. But it feels like the best out of all the gameplays.
Yeah, I loved the way the karts 'felt' in SC, even moreso than 64. Also, SC has the unique distinction of being the only game where Red Shells have an effect when tossed backwards. It would just lie there motionless until someone got into the vicinity of it, in which case it would lock-on to the person who "woke it up" and hit them. It was a neat concept, and it kinda stinks that they just up and scrapped it like that.
On the other hand, the coin-collecting never really sat well with me. With so many carts in close proximity, you were bound to bump into people quite a bit. Considering the fact that you lost a coin every time someone bumped you, and you automatically spun out after losing all of them...yeah. That got annoying really quickly. Thankfully you could turn them off in VS mode, and there were plenty of courses where you could collect a bundle of them so you could unlock the SNES courses.