...well, as you gain experience you WILL see them! ...especially if you graduate to more advanced diving, liveaboards are pretty big on them and for many dive ops they are REQUIRED safety gear...as in, no SMB = NO DIVING !!! Probably they are not really mentioned in OW class because it's not politically-correct to point out to brand new divers the risks/dangers of diving, the industry needs to have you spend $ on gear, not potentially scare you off !
When you start to run with the big dogs (doing remote, middle-of-nowhere liveaboards...my fav!) you will become much more aware of safety/self-reliance/redundancy.
Here's my standard 'kit' with respect to safety gear, on ALL dives :
2 handheld lights
1 shoulder mounted (military grade-NATO approved) emergency signal strobe from JOTRON
1 signal mirror
1 knife
1 line cutting tool
2 6' Halcyon SMBs
1 spare Atomic Aquatics mask (in BC pocket)
1 air-horn (powered off BCD hose)
1 Nautilus Lifeline emergency radio/GPS unit (on order for delivery April 2011)
(also all dives with at least 2 computers, sometimes 3...plus SPG......stainless steel fin spring straps which can't break......Oxycheq EXTREME 'wing', the strongest/toughest wing material in the world!)