order of Kata

[Zaine]

Based on this my question is: Why should Goju Ryu Karate-ka spend much time on a concept that wasn't important enough for the founder(s) of our art to be included in our core Katas?

Whereas I am not a Goju Ryu practitioner, I believe the answer to you questions is that times change. We're in a different world than the founders of most of our respective systems and with that sometimes techniques need to get added and subtracted from our core practices. That's just my two cents.

[CredoTe]

... Kihon works well moving forwards and backwards but doesn't really incorporate turns.

The taikyoku Kata does help with the more advanced Kata with manoeuvering and hip movement.

Kihon Ido (fundamental techniques in motion) allow for turns and so do partner exercises. Especially, because the turns in Taikyuko are wrong from the Goju Ryu perspective, as I explained before.

Stepping into zenkutsu dachi with an oi zuki you do have a concept of distance. You can step into an attack and zenkutsu dachi is a stable stance.

Punching over long distance is not included in any of the other Katas (that contain the core concepts of Goju Ryu). Neither is Oi Zuki. I just did all Katas and checked. Really. There are only 5 Oi Zukis, one in Sanchin and two in each of the Geckisai forms, which are fairly new themselves. Oi Zuki in Zenkutsu Dachi is a Shotokan concept (appears for example in the Heian Katas, and sporadically in advanced Katas like Jion, Gankaku etc..). But not nearly as often as time is spent on practicing it in Kihon. Based on this my question is: Why should Goju Ryu Karate-ka spend much time on a concept that wasn't important enough for the founder(s) of our art to be included in our core Katas?

(I play the devils advocate here; for my Nidan in Shotokan I have done a lot of Zukis in Zenkutsu Dachi)

Like Zaine, I am not a Goju Ryu practitioner, but I think the path to the answer lies in your question: concepts. What is the Goju Ryu concept behind Oi Tsuki? In Matsubayashi-Ryu (Shorin-Ryu), we do not prescribe to the notion of a "long distance" punch, either; we are not "long fist", if you will. However, we do have Oi Tsuki; our concept is that it is to be used in conjunction with Tenshin (body positioning/movement/footwork) as a counter from a distance of arm's length or slightly more in order to engage in Iri kumi (in-fighting). It is not a reaching punch; the Tenshin is absolutely necessary or it will fail. We practice Oi Tsuki in both Shiko dachi and Zenkutsu dachi...