how important is it to replace a timing belt
It is only important to replace the timing belt if you want to prevent the engine from self-destructing when the belt breaks. That is why it is listed as a vital maintenance procedure for your car.
Replacing the timing belt on schedule will likely cost you about $400–$600, depending on where the job is done, but it is recommended that you also replace the water pump, the serpentine belt, and all belt tensioners at the same time that the timing belt is replaced in order to save on labor costs. All told, these procedures might run you as much as $600-$800, again depending on where the job is done.
By contrast, if the belt is not replaced on schedule, the damage to your engine when the belt snaps will likely cost somewhere north of $2,000 to repair. The choice is yours.
If the engine is interference then very. Either case(if not interference) it will leave you stranded ANYWHERE without warning.