You may be an exceptionally charismatic person that can get any job you want, and if so, you can ignore the blog post and my responses. Think about it though, are you working at your dream job? Is there somewhere else you really want to work? Wouldn't it suck to choke at that interview?
For me, part of the challenge with interviewing is remaining calm under pressure (your future depends on how you answer the following question...). After a couple interviews recently, I found it got a little easier to handle.
Interviewing is also a matter of selling yourself, and this is more than just answering questions or solving problems - you also need to be asking your interviewer good questions. If you lead them the right direction they can come up with answers for why you're the best person for the job. (Asking the right questions can also help you determine whether you really do want to work at a company.)
There are also plenty of other sites, usually with a competitive edge, like HackerRank or TopCoder. Doing problems like these really is not a waste of time even if you're completely content with your job. While a lot of it may seem like algorithmic trivia, you'll internalize it sooner than you might expect, and then you'll start finding real-world uses for some of these tricks and, more generally, your "mental overload" when thinking through a piece of tricky code will gradually go down and down.
Re: interviewing frequently just for practice, that's great if you're in an area with an abundance of tech companies. I think that could be risky if you're in a city with just a few that you'd want to work at, though. You don't want to bomb an interview at the only other company in your city you'd want to work for just because you were rusty and used them for practice.