Its bad practice if your concern is readability in your mark-up and maintenance, especially on a larger scale it can get quite messy - also keep in mind the inline JS will never be cached like an external js file would so you do suffer a bit in regards to performance especially if you abuse it
Read this article for more insight: http://robertnyman.com/2008/11/20/why-inline-css-and-javascript-code-is-such-a-bad-thing/
