Mep,
From one point of view, you are right: Spectral gear, through a technical explanation plus esoteric mix, forces you to get into the system full monthy.
Notwithstanding, people go with single-branded amplifications in many further cases: think about Burmester, Naim, McIntosh, Kondo, Audio Techne, Dartzeel and similar. For many candidate buyers, knowing that staying homogeneous within the brand is a high performance warrancy is a purchase quality reassurance.
I don't own any Spectral gear, since, unfortunately, I cannot afford it. But, even if I had unlimited amount of money to spend on an audio system, a Spectral amplification would be my #1 choice (followed by Burmester and VTL). Notably, Spectral represents an "affordable" super-hiend company, with reference products that cost just a fraction of the price of other brands.
To be an owner of Spectral gear requires a special type of owner who is willing to put up with marginally stable electronics that are basically unsuitable for use with any other brand of electronics and cables. Mixing and matching Spectral amps and preamps with other electronics is like playing Russian Roulette. Who wants to put up with an amp that shuts down all the time when you are trying to listen to music because it doesn't like your electrostatic speakers and/or your non-Spectral preamp?
If I remember correctly, Spectral is not the primary business of Rick Fryer. His primary business deals with RF electronics which partially explains why he builds audio gear that is operating into the MHz bandwidth. From Spectral's website: "Uncompromising audio circuitry must be extremely wideband, but wideband output can broadcast like a transmitter, to produce megahertz oscillations which ring and distort the audio signal. To prevent oscillations in the Spectral system, conventional "wire" cables have been replaced with precision tuned cable "networks" developed by MIT. "
If you don't use Spectral gear with other Spectral gear and you don't use approved Spectral cables, you are at the constant risk of sending you gear into high-speed oscillation. That is why I said before-can you really own drink half of the Spectral kool-aid? If you buy into their philosophy and love their products, how do you go against their preachings and use their gear with other gear and other cables that are going to cause suboptimal performance if not ultimately damage that requires an expensive repair and an "I told you so" from Spectral? To me, if you are going to join the Spectral church of audio religion, you need to be "all in" or risk be excommunicated from the flock and having lots of downtime with your system.