How often does evacuation happen as shown during the pre-flight safety videos/stewardess instructions?
I didn't do statistics, but The Aviation Herald lists units to low tens a year worldwide.

Most of them happen at airport due to fire indication (sometimes even false) or smoke on board.

On the ground, fire spreads a lot faster than while flying, because when flying, the wind is too strong and blowing the flames off. The most notable example would be China Airlines B738 at Okinawa on Aug. 20th 2007, airplane in flames arriving at stand. The aircraft was already at the gate when the flames were first noticed, evacuation was immediately initiated and by the time everybody left, it was already engulfed in flames, about 3 minutes after the fire started. While China Air might not be "western" in strict sense of the world, it is largest airline on Taiwan and follows the same standards.

Another notable accident is Egyptair B772 at Cairo on Jul 29th 2011, cockpit fire.

Here are some recent evacuations mentioned on AvHerald:


Air France B773 at Paris on Jul 28th 2013, evacuation while boarding due to fumes
US Airways B752 at Kona on Jun 8th 2013, evacuation during push back causes 4 injuries
PSA CRJ2 near Philadelphia on Apr 17th 2013, smoke on board, evacuation
Easyjet A319 at Belfast on Jan 6th 2011, taxiway de-icing prompts evacuation


Notice, that they are all smoke or fire events. For some other reasons, see e.g.:


Wizz A320 at Rome on Jun 8th 2013, left main gear did not extend
Alitalia A320 at Rome on Sep 29th 2013, right main gear did not extend


two similar accidents where both aircraft were evacuated via slides. The reason in these, and many other gear collapse incidents, is that they don't want to wait in a damaged plane where there is a risk of fire, plus the difficulty of getting stairs to the aircraft resting in strange attitude.