Should I use UPS with my laptop to extend the battery life?

No. External power alone is not enough for all cases. Many laptops only work at full capacity if they have a battery and external power. This is because the external power supply can only provide 70-90W but the machine may need 100+W when CPU spikes high and the HD/DVD is spinning. Having the battery on board allows the machine to sip from the battery when needed and charge it back up when not.

Battery longevity is always a debate. Personally, I wouldn't bother using a UPS and storing the battery away. What's the point? A battery in a laptop basically is a UPS, in a way.