Just a quick case that came in the other day. There was decay in the pit on the side of the tooth which had penetrated all the way to the metal filling in the top of the tooth. These old metal fillings also swell and corrode over time. With chewing and cycling of hot and cold food/drinks, the teeth can crack. (you know – that wonderful uh-oh moment while eating crackling at Christmas). By the way – note the old white resin sealant on the tooth behind aimed at preventing getting the decay in the groove in the first place.
Tooth coloured resins are ideal in this situation. They bond to the tooth – especially to enamel above the gum, and help restore the tooths former strength. They may have to be replaced more often than the old metal fillings due to chipping or wear – but generally technology has advanced to the point where these are the first choice material for such cavities.
Apart from that they look great.