I’m not going to list reasons why you should wear your seatbelt.
I’m simply going to show you what happens when you DON’T wear your seatbelt.

If you’d like to have an impression of your face on the steering wheel at 30mph, thats up to you.

And, if you must have them :

In 2005, 43,443 people were killed in the estimated 6,159,000
police-reported motor vehicle traffic crashes, 2,699,000 people were
injured, and 4,304,000 crashes involved property damage only.

It is estimated that seatbelts saved 15,632 lives in 2005.

(NHTSA)

Seventy-three percent of the passenger vehicle occupants who were in a
fatal crash in 2002 and were restrained survived; of those who were not
restrained, only 43 percent survived. [NHTSA, Traffic Safety Facts
2002, October 2003]

In fatal crashes, 73 percent of all
vehicle occupants who were totally ejected were killed. Only 1 percent
of restrained occupants were ejected. Safety belts are effective in
preventing total ejections. [NHTSA, Traffic Safety Facts 2002 –
Overview, July 2003]

In 2000, safety belts prevented nearly 11,900 fatalities and 325,000
serious injuries, saving $50 billion in medical care, lost
productivity, and other injury-related costs. [NHTSA, Economic Impact
of Crashes, May 2002]
In the year 2000, the total economic cost of
motor vehicle crashes in the United States was $230.6 billion. This
represents an amount equal to 2.3 percent of the gross domestic
product, or $820 for every person living in the United States. [NHTSA,
Economic Impact of Crashes, May 2002]

The lifetime economic cost to society for each fatality is over
$977,000. Over 80 percent of this amount is attributable to lost
workplace and household productivity. [NHTSA, Economic Impact of
Crashes, May 2002]

[Ed. comment – Even if YOU wear your seatbelt, and your buddy DOESN’T]

Overall, those not directly involved in crashes pay for nearly
three-quarters of all crashcosts, primarily through insurance premiums,
taxes and travel delay. In 2000 these costs, borne by society rather
than by crash victims, totaled over $170 billion. [NHTSA,Economic
Impact of Crashes, May 2002]

Stats from THIS page.