Projectional editors have always fallen behind plain text ones when it came to manipulating parentheses in expressions. Adding a single parenthesis would turn the AST invalid and thus you had to add both parentheses at the same time through actions or intentions in order to preserve the AST structure.
Not all is lost on the usability front, fortunately. As a small teaser for the upcoming MPS 3.1, check out this short video on parentheses projectional editing and see how parentheses editing in BaseLanguage has been enhanced.
Develop with pleasure!
-JetBrains MPS Team