Couplers are a very ancient device in the pipe organ, and their origins stretch back at least 400 years. We may never know when the first coupler appeared, but its antecedents are visible In Renaissance paintings which show a latch-down lever to hold the bottom note of a keyboard for a drone bass. It was not long until a pedalboard appeared which was permanently coupled to the manual keys. Eventually, separate independent pedal ranks appeared which made it necessary, for the first time, to install a coupling action so the organist could turn the coupler on and off.
When multi-manual organs appeared, it was natural to add manual-to-manual couplers. Though couplers became common, many tracker organs were built from the 16th century to the present day with a manual division isolated from the others, playable only by itself, with no couplers to any other manual or pedal.
Tracker organbuilders occasionally pursued the use of octave couplers, and one encounters 16' or 4' couplers on some instruments, including a few built today. The use of octave couplers in tracker action was always accompanied by the problem of a heavier key action, and such couplers were usually found only in small instruments.
A form of coupler mechanism was actually used to construct a unified tracker long before the use of electricity in organs, but such mechanisms were extremely rare. The validity of such an approach was illustrated to me by a 19th-century statue of St. Cecilia which showed her holding a tiny one manual pipe organ in her arms. The front of the organ had six drawknobs!
The development of the concept of "full" couplers (16', 8', 4' couplers between all manuals and within all manuals, and 8' 4' couplers from all manuals to pedal) accompanied a new tonal design for the organ which eliminated mixtures and upperwork. The 4' couplers were supposed to take the place of the newly missing upperwork. Thus, couplers appeared just as they were needed to contribute to new tonal styles, and the new styles became completely dependent on the use of the full-coupler scheme: an interdependency which would not be understood by later rebuilders who would discard "extra" couplers as a means of purifying earlier organ designs. Many coupler-dependent organs would suffer as their only means of fulfillment disappeared and they were left with only unison couplers.
In the coupler-rich era of the early 20th century, some bizarre examples of coupling appeared. This was particularly so in the world of the theater organ, where non-unison couplers were utilized in some designs. We can wonder at the technical feats which would be possible with couplers such as Solo to Great 8', 62/5', 51/3', 44/7' and 4'. Even a large church organ might have a Pedal to Pedal 4' coupler, complete with extra pipes. Such things are now rare.
As the new tonal designs of the late 20th century gained favor, the reappearance of mixtures and the tonal completeness of divisions made "full" couplers less necessary. Both intramanual (Great to Great) and intermanual (Swell to Great) couplers declined in numbers, and the organ once again returned to the simple unison couplers which it once had. As organists and builders, we have not always understood the full implications of couplers and why some builders used them in a particular way, or even repeated their use from some other source without any apparent thought.
Many builders realize that excessive coupler use creates tonal problems, and they, therefore, install couplers reluctantly. It is true that with the best of modern tonal designs there are severe problems of scaling and voicing if the builder must allow for the presence of 16' or 4' couplers. Where such couplers were a necessity for early tonal designs, they now cause only trouble and tasteless excess. In many cases, a coupler is far more responsible for a certain tonal effect in an organ than any other given stop.
I think it may be true that organbuilders worry more about the inept use of couplers than any other device in the organ, and the limitation of them to the barest minimum is a way of relieving the anxiety about their misuse. On the other hand, with a skilled organist on the bench, the proper couplers pave the way for stunning and unpredictable surprises. They are a blessing and curse all at once.
It may be true that couplers and unification are "cheap" stops for the organ, but in many designs it is necessary for the builder to try to develop flexibility with a confining budget or space. It is also true that builders have now developed a more cautious approach to couplers, and their presence must be totally justified in the organ by a thoughtful design philosophy rather than the mindless use of the every-coupler-everywhere approach of the past.
Whether tracker or electric action, organs do depend on couplers, and they will be a part of most multi-manual and pedal instruments. The last word on couplers has not been written, and most builders would much rather install pipes than couplers, but the contemporary pipe organ (unless it is unified to the extent that couplers are no longer needed) will have couplers. The thoughtful builder will include those couplers necessary for the successful use of the organ; the thoughtful organist will use those couplers with musicality and taste.