I think the difference between a twice-monthly set-up and sending extra principal payments is that the twice--monthly is actually two payments of principal and interest that equal up to what you would have paid anyway, it's just split, so you're paying less interest by making half your monthly payment two weeks early. (depending on how it's structured you may also be making 26 half payments instead of 12 full ones, further decreasing the balance.) Making a principal-only extra contribution saves more in the long run, but it also increases what you pay in a month. E.g. if you have a twice-monthly, you're sending in two payments of $320 each, but if you have a standard monthly payment you pay $640 plus whatever extra you choose to send. To repeat myself, sending extra principal is better for paying off the mortgage, but twice-monthly is still better than nothing.
ETA: those fees! Convenient holdover excuse from the days when those extra payments had to be processed by hand. Loada rubbish!
-- I once sold a house OWC to a sweet young couple who were so enthused when I said the rate was 8% and the payment was $1210 a month. He bent over the calculator for a minute and grinned. "That means we'll be paid off in 12 years!" ??? They thought I was the scam artist of the year when I told them no, 8% is not a one-time fee, it's the annual cost of the loan. Ouch. I guess you don't know til you know.