If you have an interest in academic positions this is a golden opportunity that you should not pass up. You will have actually taught some reasonable portion of a class in your field and will be able to discuss with the interview committee how it went, what you would change, how you would teach such a class at your prospective institution. It will go a long way in helping you write your statement on teaching philosophy. It also will let you know what you are in for - preparing lectures the first time through is an incredibly time consuming task.
If you have no desire to be in academia, well, that is something different.
Frankly, I think your adviser is being very nice to you.