Yes, it is a waste and really a bad practice but this is the only way to get into these big cos unless until you are famous speaker or github\twitter\youtube rockstar.

I know a lot of interviewers who go through cracking the code interview book before taking an interview so what is the point? It is like school system. You read from a book and in the exam you will get questions from that book. It is really a pathetic and sad approach. Some people even spend 3-6 months practicing all of this. I would rather spend all this time in building something new.

We code in our zone and peace of mind. While interviewing, you are supposed to keep talking what you are doing and thinking. How is it possible? I think it is extremely hard to come up with a solution on the spot unless until you have spend long time practicing these puzzles then you get accustomed to these tricks.

If you are still keen on trying then there is an easy way - telephonic rounds are mostly about arrays and string so you can focus on that. For in-person rounds, you can focus on graphs and trees. It may or may not happen for you.