Seems like a silly question at first. But if you think about it, it becomes a bit of a paradox. Every other musical instrument I can think of makes sound by itself vibrating or contorting the air around it in some fashion. We can record that performance and then re-simulate the experience thru speakers or headphones. With a synthesizer, its initially the speaker or headphone making that sound, simply directed by a series of electric pulses.
Of course I totally love the electronic medium. I suppose I would defend the synth as we know it against nay-sayers (they are still out there) as certainly being an "instrument" though within its self, it may not contain "musical" quality. It still is in effect acting as the pick, hammer, stick, or breath just like any other instrument depends upon. So this reliance on switches and circuits triggered by keys to set speakers in motion instead of using a reed or vibrating string is a big difference.
In summary, I guess what I'm saying is that any synth is perhaps only half of an instrument until you include the speakers in the discussion and maybe the amp too. Of course that can really color a sound good or bad.
Unfortunately, the love of HI-Fi systems seems to be on a rapid decline since the 90's. We're lucky if somebody listens to your music thru some decent earbuds. TV sound (unless you have a surround sound system) is usually as just as poor as a computer speaker, and car systems are usually very filtered. If you think about it, only a small part of the population is able to reproduce good sound as it was intended to be heard, and it seems the the interest in doing that is becoming less, not more.
We in the synthesizer community are fortunate out due to the nature of the beast to be at the forefront of audio prowess. It's really half our instrument after all.