A realistic and valid systems test and evaluation would be to create an artificial RF environment consisting of real-world interfering transmitters using various modulation formats summed together with a weak wanted signal at suitable frequency separations.
I cannot see any case against such an Modus Operandi, as transmitters/transceivers are a "dime a dozen" compared to high-performance RF signal generators.
A suitable way could be to establish a wanted signal reference level, for example 12 dB SINAD in an SSB bandwidth, and after this turn on one interferer after another and notice how much the levels of the interferers need to be increased until
degradation of the SINAD commences as a function of the number of interferers, their power levels and frequency separations.
This would mimic "real-world" conditions very closely, and if the number of interferers is large enough (say 10), even contest or pile-up conditions could be emulated.
When transmitters of varying quality are used as sources, there is a definite possibility to evaluate the system impact from the spectral properties (wideband noise, phase noise, "splatter" and keying artefacts) from "low-end" as well as "high-end" transmitters.
And if you used the average ham transmitter as the signal generator you will soon realize the realities of air receiver and transmitter performance.
Use a clean signal generator and measure the receiver detecting the IMD product at level X. Now use a typical ham transceiver as the signal generator and watch the IMD product disappear because the transmitter is so filthy. That is the real world ham radio on air test lab. Ignoring transmitter performance is technical stupidity at its best.
We dont transmit with perfectly clean signal generators we transmitter with crud generators. Since there is no splatter blanker, why worry about receiver performance when the receiver has probably 50db or more headroom before its even stressed. Receivers in the real world get stressed by splattering hams, not ultimate IMD dynamic range problems.