I definately agree that a synthesizer emulating an instrument will never sound exactly right. However, I don't think people have lowered their standards to the point at which they think a bad synth string emulation, or something of the sort, is acceptable. I just don't think people mind synth instruments, especially since they are a cheap alternative to adding an extra musician to your band, for example.

I don't use synth emulation of orchestra/band/etc. instruments for anything except composition, in which case I use them so I can hear a reasonably good representation of what my piece will sound like played by the actual instruments. Of course, the quality of this representation depends on your synthesizer - and I think many professional synths by Roland, Korg, Yamaha, etc. do the job rather well.

Most of what I do use my keyboard for is the electronic voices that only a synthesizer can produce - waves, pads, etc. These, in my opinion, are the true beauty of a synthesizer, and besides, they just sound cool. I agree that their best application is "new sounds," as you put it (this is how masterpieces like "Jump" are born), but I still think that their emulation of "real" instruments can be really quite good.