My medium for sculpting is polymer clay and plastic is what it really is, vinyl to be more exact. The question of its validity as a fine art medium is bound to come up. So I'd like to just expound on it a little.
Beyond its value to me as an art medium (which is great because it's so forgiving) it has durability. Sometimes beyond traditional mediums...although it's susceptible to UV rays it's not an issue if cured well and it's slightly flexible (un-noticeable in larger areas) so it won't chip or shatter. It doesn't feel cheap, it feels rich like stone once formed and protected by a suitable varnish. It's got a better feel than fired clay in my opinion. And it's not been poured like bronze, it comes to you fresh from the artist's hands. Granted, bronze sculpture started in the artist's hands, but every process artwork goes through creates minor changes that weren't left by the artist.
And not only is it moldable by hand it can also be carved and sanded if crisp detail is the goal. So it does fit in with stone, wood or ivory (and other carving materials) more than bronze. I will admit that more skill is involved in the carving aspect of those materials, once you've taken something away you can't put it back. With plastic if you do break a thin appendage while sanding or carving, even after its cured, you can re-bond it with new plastic, re-cure it, and it's even stronger than the original bond because it chemically bonds into a single piece again.
So like any newer medium it comes down to perception. I think questioning the material quality of a medium is a valid process when buying art, and I think plastic is a good bet for longevity. When it comes down to bragging rights though, it might be harder to say "This is made of plastic!" rather than marble, bronze, ivory, etc. And I guess that's a valid point.
So what does an artist do? Well, then the point becomes more about creating something that is so good that someone will say "Wow! Can you believe this is made from PLASTIC?"
In the end, as long as an artist isn't using shoddy materials of any kind, the artwork should speak for itself. And then it becomes a matter of a collector wanting the piece because of its artistic value.