Is leaky valve dangerous?

The leakage may be mild, moderate, or severe, so the answer depends on the degree of the leakage.

Mild or slight leakage does not need any manipulation you can live with it for years without any problems.
Moderate leakage you can also live with it but it needs follow-up by echocardiography every year.
Severe leakage you feel tired, off breath, and swelling in your legs and tummy due to impairment of the forward flow of blood and hence heart failure, here you need intensive medical treatment by water bills while we prepare for surgical correction (repair or replacement).