There’s a difference between putting the Lord in a dirty feeding trough because it’s all you have and doing so for your own convenience....
But plastic cups are humble!!
Some might argue that Jesus’ humble birth and life were not incidental, but intentional.
As for me, I don’t really care what vessel is used, as long as Communion is celebrated respectfully. After all, Scripture is clear that it’s “bread” that we eat (to use a term Paul used, http://www.biblegateway.com/passage/?search=1%20Corinthians%2011:27-28&version=DRA and elsewhere), and not corpuscles.