Also, you could setup a simple email to SMS gateway, so you can get a text message whenever somebody emails firstname.lastname@example.org.
In a nutshell, your phone battery will drain quickly only if you keep using it as a smart phone, i.e. using data, wifi, bluetooth, having your screen on all the time, etc. If you keep a dedicated mobile phone for emergencies only and use it primarily for texting, you will have all the benefits of a pager while remaining in the 21st century.
You can push a dumb phone battery to a full week if you do it right, and to me at least, charging a phone over weekend or in the car is easier and cheaper then swapping batteries.