Well, I'm not saying you'll necessarily NEED them, but backups are always good. A couple of my parts were bad. It happens even with the most careful quality control. If I'd ordered extra parts in advance I'd still have been able to return the bad parts, and Id have the use of the backup. Besides, some extra batteries are always useful, and you can also get the extras in different colors to increase your options. Like I said, IF you can afford it.
I'm getting a full assortment of extras, about the same as a new kit minus the USB, charger, and extra switch. It comes to less than the full set. SO I'll have four batteries for each model, good for when I travel, an extra atomizer just in case, and an extra switch, probably the manual one, also just in case. And since I ordered the Titanium Classic and the Black Yentyl, I'm getting the extras in black Classic and Silver Yentyl, so essentially I'll have a second set in a second color.