Plug-ins do not modify jQuery's core. They extend from it.
Agree. Except that the number of the different plug-ins is finite, while the coder or client supplementary needs are not Usually, the client wants an extension which is not available as plug-in, thus you have to code it somehow alone. One of the Murphy's Law, I presume A corollary is that the client usually wants a small (he thinks) extension at the end, after you have used a plug-in or another, and things can not be changed in the manner he wants, unless you start everything from the beginning