This is both very true and a bit false. My 3D printer sits idle more than 95% of the time, but I just could not have done what I do unless I had one right next to me so I could iterate quickly. I usually need to do 4-5 iterations of something to get all the tolerances and details right, which cannot be scheduled a week in advance.

However, if you can share a printer with someone and have low-latency access to it, definitely do not buy your own, just use that.