What you are describing is extremely common. Brown streaked enamel, especially vertical lines, is a common finding in teeth which have undergone a lifetime of use (chronological or accelerated through misuse). The streaks are simply stained stress cracks, which are often contained in the superficial enamel. They are not true fractures of the teeth - simply a sign of ageing. They are most unlikely to cause any issue other then aesthetically. This is most likely why your dentist has not mentioned them.
Only your dentist can advise you as to the advantages and disadvantages of having crowns or veneers placed on these individual teeth.