I don't mind using a library for dom manipulation, or for xhr, or for missing ES5 function ... My problem with jquery is that it does all of it, in a very entangled way. This makes it very tempting to mix parts of your application that really shouldn't know about each other (like dom manipulation (UI) and XHR (IO)). I prefer to use a dedicated library for all of those things and to hide the fact that I am using a particular library to the rest of the application. I've seen standardising on one true huge library throughout an application gone wrong several times and it's very hard to refactor your way out of it.
