Can anyone tell me is tooth/enamel demineralization bad ?

Yes: Demineralization is the process where the tooth enamel is being destroyed and broken down. Demineralization and remineralization is a dynamic process that is mediated by your saliva. Frequent acid exposure will tip the scale to permanent loss of tooth enamel, ie decay, acid erosion, white spots. Minimial acid exposure is important to maintaining strong healthy teeth.

Of course: The demineralization process results in the loss of mineral content from tooth enamel and causes decay.