"When should I use the Magento Compiler"
If for some reason your system does not have a PHP Accelerator installed ( unlikely but possible )
If your running a very old version of magento ( you really shouldn't be )
Why should you keep it disabled ?
I totaly agree with Fiasco Labs comment above.
While you didn't give your magento version number, this is true for all magento (CE) versions over 1.6.1, if you enable the compiler on a system that already has a php accelerator ( like PHP-APC etc ) your entire store, from admin pages right through to a customer checking out will be measurably slower, worse yet , many of the plugins/mods you find at the magento connect will not function correctly with compiler active.
In short, in 2014 there is never any good reason to enable the compiler.