A couple of members have prodded me to post my thoughts on this thread, so here goes now that the busy week is behind me. Obviously, I have a commercial interest and will try to not explicitly talk my own book but feel free to take it with a handful of salt etc......
To get the first question out of the way, it should come as little surprise that a cable maker might choose to use a full loom of their own products. To do so is both practical and helpful for ongoing product development. So far, so good but why should others take this approach, or indeed why might they choose not to do so?!?
Taking a step back, I feel it would first be helpful to rationalise on what qualities might reasonably be expected from any hifi product and, hence, desirable from a cable loom. “A straight wire with gain” was a suggestion proffered many decades ago yet continues to serve us well today. Any piece of hifi gear should pass through the musical signal with minimal losses or adulteration. It isn’t called ‘high fidelity’ without reason. But what qualities should this deliver in practice?
Transparency would seem an obvious virtue and this is something which imo one can never have too much of. If you don’t like what you’re hearing, then there is likely a shortcoming elsewhere being revealed. Whilst a cable can’t add any information, it should certainly aim to lose as little as possible. All those little low-level details and textural elements recreate the ambiance of a recording and help bring the music to life.
Neutrality has been suggested by others and this quality is often used in the context of tonal presentation (warm / bright etc) but can also be applied to the evenness of distribution across the frequency range. The absence of colouration is a critical part of achieving this and should ideally be innate to the wire and plugs rather than achieved by offsetting opposing colourations.
At this point, we reach a fork in the road. Those who want to use cables to tune out the innate characteristics of their systems are actively seeking a colouration and can turn off here. Those souls seeking unadulterated high fidelity can carry on straight for a while longer until they reach the next junction on ‘loom highway’.
Can your prospective cable supplier actually supply all the cable types you need? Everybody needs power / interconnect / speaker cables but what about usb / spdif / lan / tonearm / dc etc? And, if so, do all their product range offer similar levels of performance? If we can tick these boxes, you have a sensible basis for considering a full cabling loom from them. But why should you consider this?
An underlying assumption about a single-brand cabling loom which is genuinely transparent and neutral, is that the various cables have been developed to operate in harmony with each other. Presupposing their designs exhibit ‘regular' electrical parameters, this confers an encouraging level of confidence that the pairing will be successful for most users, both electrically and aesthetically. Also in concentrating more business with a single supplier, you should be able to secure more attractive pricing and develop a deeper relationship for future advice / support etc.
Now to look at things from the opposing perspective; why might one not consider a single brand loom?
The ‘cables as tuning devices’ tribe have already been mentioned above and it is implicit that they will source from different suppliers.
I can also see how inertia might be a significant factor for many who have gathered a collection of cables over their ‘career’ and would sooner avoid the hassle of change. Products do however evolve over time - I know this first hand from developing my own range - and it may now be possible to get something better for less than the resale value of what you already have.
Then there are those who are (rightly imo) suspicious of industry pricing practices, which are often at their worst in the cable segment where list prices can be extravagant yet dealer cost might be as low as 25%. Some audiophiles may choose to bide their time and wait to buy a few years later on the used market, though it will take quite some patience to put together a complete loom with this approach. Such is the lot of the Veblen goods worshipper on a budget!
To conclude, there are worthwhile objective reasons to pursue a complete single brand loom but also understandable, more behavioural and circumstantial, reasons why people often don’t.