This close-mindedness has a very simple reason:
Only 10 years ago, a good MIDI orchestra was simply a technical impossibility. MIDI was certainly good enough to create your own rock sequences, or to back up a country singer, but realistic orchestral sounds was pure science fiction at the time.
What's worse, many MIDI users did not have the courage (or the ears) required to admit these limitations, and tried too often to "impress" classical musicians with cheesy MIDI performences of orchestral works.
It takes only one listening to Beethoven's 5th on an old soundblaster card, to make a MIDI non-professional hate this thing... And when the tech geeks boast at how "good" the sample sounds, it doesn't help either.
Of-course, times has changed. Today all one needs is an average sound card (the Audiophile really is an overkill for this purpose), and high quality samples which are freely available on the internet. But most people who heard the horrible MIDI of 10 years ago is not likely to give this medium a second chance.
Another problem is, that even today many MIDI amateurs (and some pros as well) don't realize the kind and amount of work needed to create a good classical music sequence: Many don't appreciate the need of very high quality samples (the ones coming with the average soundcard are roughly 100 times too small). Niether do they understand that it is their job, rather than their computer's, to supply the human "feel" for the music - either by recording it live from a musical keyboard, or manually changing the tempo and dynamics in the MIDI file itself.
The result of this ignorance is, quite often, monotonic cheesy MIDI sequences which are only a trifle better than the ones existing 10 years ago. And sadly, most MIDI music found on the web today is of this low quality, thus perpetuating the myth of "MIDI orchestras always stink"