Nope, thrips are not as bad as spider mites. They cause less damage unless you have a REALLY bad infestation, but the damage is easy to spot early and deal with. By the time you see mite damage, you're already fooked.
Also, thrips are easier to eradicate, as they are susceptible to a variety of insecticides that are not very harmful to humans, such as pyrethrin. And their life cycle includes a stage IN the soil, so you can use physical barriers to interrupt it.
Where do htey come from? Shoot, damned if I know, hell maybe?