Natural oak cork is a miracle of nature. The cork bark cells resemble honeycombs and are filled with air. This structure allows them to shrink to half the size without loss of flexibility. Also, natural cork has elastic memory – when it is pressed, it tries to restore its original size. Thus, it expands in the bottle neck and tightly closes the bottle for decades.
Natural oak cork is not the cheapest material to manufacture. Therefore, in Spain, France, Italy, and especially in the New World, more and more bottles are closed with colorful silicone stoppers or screw caps. For wine lovers, this has become a norm already.
To summarize: plastic stoppers or screw caps do not make wine less valuable. A judgement on the quality of the wine based on the method of its closure is impossible.