TA is a great training for presentation, communication, management, and personal skills. You have to be able to describe course materials in a way that the students can understand, and you have to balance their learning and happiness. Naturally, students want to minimize workloads while maximizing grades. Dealing with a large class (hundreds of undergrad students) is not unlike managing a mob.
It is great if one can focus exclusively on research. That is what I prefer then as a grad student and now as a prof. But without sufficient communication and personal skills, one cannot succeed even with great research skills.
To start with, a great idea is of no value if it cannot be understood and appreciated by people.
Thus, I do not consider TA a waste of time. Quite on the contrary, it is an indispensable part of research training.