because...
1. Sometimes you have to "play all". A good SD/DD game is an example, where you can't continually bail out when the count tanks. Fortunately it doesn't tank long as the shuffle is always not far away, but if it does, you will have to keep playing.
2. Because of (1) above, the negative indices are important, in that they are the most profitable of the negative indices, giving you a way to improve your EV in a negative-EV situation (negative count).
If you are a 6-decker (shoe), and wong out at -1, then there is no need to remember negative indices. If you plan on playing games where you can't continually "sit out when the count tanks" then those indices can help.