Jan 2018 UPDATE: When I answered this question last year, I neglected to mention a third way to access Google APIs with JavaScript, and that would be from Node.js apps using its client library, so I added it below.
It's Mar 2017, and most of the answers here are outdated -- the accepted answer now refers to a library that uses an older API version. A more current answer: you can access most Google APIs with JavaScript only. Google provides 3 ways to do this today:
When using the REST API, you need to manage & store your source code as well as perform authorization by rolling your own auth code (see samples above). Apps Script handles this on your behalf, managing the data (reducing the "pain" as mentioned by Ape-inago in their answer), and your code is stored on Google's servers. But your functionality is restricted to what services App Script provides whereas the REST API gives developers much broader access to the API. But hey, it's good to have choices, right? In summary, to answer the OP original question, instead of zero, developers have three ways of accessing Google Sheets using JavaScript.