Is it okay to keep dentin on your tooth exposed?

Yes: Many people exhibit gum recession which exposes root surface dentin after the covering layer of cementum wears away. Sometimes this results in tooth sensitivity which must be treated but often it is asymptomatic. It can pose an esthetic challenge but not always. If it isn't troubling it can be fine but keep in mind that dentin is while hard, much softer than enamel and is more prone to wear/decay.

Not really: Dentin is softer than enamel and will decay or wear away quicker than a tooth that is properly restored. There are certain situations where dentin is exposed and no harm occurs, but each case is different and needs to be regularly monitored.