I agree with your sentiment, and I prefer the mathematical definition of Big-O to hand-wavey descriptions, but the plain English descriptions do have a place in presenting the material, in my opinion. Once you understand the idea of what Big-O is and isn't used to measure or explain, the actual definition makes sense as a translation of those ideas into math. It's like an example that goes with a theorem in a math textbook. Yes, all of the information is technically in the theorem, but that doesn't mean the example doesn't help a student understand.