The best money that I've ever spent was my "aerobics instructor" microphone and amplifier. If you are a music teacher who is trying to teach without amplification go ahead and spend the money to get the microphone, no amount of unamplified teaching is worth loosing your voice over.
There are multiple reasons to take care of your voice with plenty of water, rest, limited talking, and all the other healthy things we are supposed to do for ourselves anyway.....Here are the few that come to the top of my head
1. I like to sing - a tired voice keeps me from doing what I love..... I hate that!
2. This is my job - if I can't do my job then maybe I need to do something else
3. My students benefit from a healthy vocal model not a tired one
4. Having a voice that is not my best voice puts me in a VERY bad mood and is frustrating
5. Singing makes me happy
6. Some vocal damage is permanent, why risk it!
7. Surgery is risky at best, devestating at worst
8. A $300.00 microphone and a $50.00 amp is cheaper than lost work and surgery