What is your opinion on proprietary scripting languages?

GameMaker has its own GameMakerLanguage and many other games and programs have their own proprietary scripting language. Do you find this to be useful and effective or do you feel that it would be better if so many software development firms just made APIs for general-purpose languages like Python? Discuss the benefits of having a custom-built language in your software versus conforming to a general-purpose, widely adopted language. Assume we are talking about any type of scripting or API-like functionality.