I don't think anyone would suggest you write all of the first codeblock every time you want to check a class. You'd put it in a function and call that, much like jquery does.
The point is that this site explains how you do it because a) that's probably easier than trawling through the jquery source b) including this polyfill when you know you need it might save you from requiring all the rest of jquery.
In that case I'd be much more likely to use a minimal jQuery replacement such as umbrella.js mentioned elsewhere here. That's 3k (vs about 28k for jQuery) so counters most of the 'bloat' objections with jQuery. I'm sure it will still irk some some purists on other grounds however.
