Today, any serious application rarely uses only one library. And very often the problem of libraries compatibility arises. Knockout MVC lacks this disadvantage. It is always possible to add your own native Knockout-code to the existing C# code (example). And Knockout itself greatly mates with any other libraries. Knockout.js doesn’t depend on any other libraries; however its core hides various useful tools for work with jQuery (that start working when it is connected). It is possible to use other libraries (for example, Prototype), but is you use jQuery (as many other developers) you can use any jQuery plug-ins (for example, jQuery.Validation, example). You can use RequireJs (example) to load big scripts.
Besides compatibility with third-party libraries, it’s worth to mention browser compatibility: Knockout MVC is completely cross-browser (since it is based on Knockout.js) and works fine in IE 6+, Firefox 2+, Opera 10+, Chrome, Safari.