I have a 160GB and a 250GB external drive and i never turn them off. I never turn my computers off either though. I believe there is a study that proved that it didn't make a difference in the life of the computer if you kept it on 24/7 (google around for it). Another option would be to go in to your energy saver settings and click on the "put hard disk(s) to sleep when possible" check box, this would for all intensive purposes be the same as turning it off as it stops the disk from spinning.
As far as the spinning life, most Hard Drives have MTBF of 1 million hours or more, thats a long time...
Ed