Very true. A lot of people don't think about this. Just making extra payments to principal each month with your 30 year loan can turn it into a 20 year loan pretty easily! A good alternative for people who can't handle big upfront refinancing costs or don't want to add years to their loan.
Some do not have a goal of paying off their mortgage--myself included--so advance payments are not for everyone--but certainly a strategy to consider. I am perfectly content paying the monthly payment--different financial circumstances dictate different strategies.