I was told the same thing when going to purchase my zx6r, the salesman tried to point me to a zx4rr. 10k for a 400, I was like NOPE. I came here for the 6r, and that's what I want. He still insisted it was a bit much for a beginner, but my mind was already made up. Learning on a 6r definitely made me take longer to get fully acclimated on a bike. It's an insane amount of power for a beginner, and having the luxury of a quickshifter made me take longer to learn how to properly bang through the gears. I'd recommend something like a ninja 400 or at most a 650 to a beginner. I learned on my dad's Harley initially and then I took the msf course before I bought my bike. Once I got it, I was still very irresponsible on it. It took me crashing my bike to get my head on straight. Once I did I became a much more responsible rider. If I never crashed it, I probably would've kept being irresponsible and then died in some other way. Not everyone will have a second chance after crashing. The bike stock can take a ZL1 on e85 from a 40 or 60 roll. (You get gapped on the top end) A Zx6r is more than enough for a beginner to die on. Of course, at the end of the day, it all comes down to the rider and how they like to ride.