It can be considered as purism (and on the emotional level it probably is), but in the background it boils down to resource management. You see, when you emulate features of other systems, you throw processor cycles at making the EP do something that is not natural for her (or is EP a he?). The result may even be quite good, but it will be seldom really good and most likely never anything close to as good as the original. And you wasted a lot of resources for the botched results.
Let's try to see an exaggerated example on the Plus/4. You have to get
YAPE and
Robocop+SID. Run the emulator and load the intro (press F8, then navigate to and select the file in the open dialog). Set up sound (Settings/Sound settings...) first like this:
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Run the intro (type "RUN" and press Enter or press Shift+F3) and listen to the sound for a while. This is the natural SID sound implemented in the ReSID emulation library. Now change the sound configuration like this:
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This is a SID emulation running natively on the machine, making the processor and TED jumping through hoops. Be prepared that the volume will be very different than before. You have to play with the volume control to find a level that won't make you deaf in one mode while still enables properly hearing the music in the other. Toggle between the two settings a few times, and you'll see how futile actually this kind of emulation is. And, actually, this is one of the better wave converters there. The thing that isn't standing out at first glance is the monumental waste of processor time (my uneducated guess is over 80%). This kind of thing leaves very-very few cycles that you could use to do other things besides playing the "music". Or you could lower the CPU consumption, but the sound quality will be even more horrendous.
The result of emulation usually is that you have to make some kind of sacrifice in other important areas just to "make room" for the thing you want to emulate. Which I don't think always worth the price.