Herkbird,
I understand that it can knock a few years off of your mortgage, but you have to look at the big picture.
In the year 2000, if you had placed an extra $100 a month on your mortgage for a 30 year mortgage you would have paid off the mortgage in approximately 23 years.
Your mortgage payment never goes down. It stays the same no matter how much extra you pay on it, so your $1000 house payment is due every month up to around 2023. Well in 2023 $1000 is worth a lot less than it is in the year 2000. So you save an extra $84,000 in house payments those last 7 years. If you invested that extra $100 a month into your 401K or another investment for 23 years, your going to have at least double your $84,000 you saved on paying off your house early.
Paying extra money on your principal of your home is not the best investment.
Idiotic is probably too strong a word. It's whichever investment each investor is most comfortable with.
http://www.bankrate.com/brm/calculators/mortgages.asp
http://www.bankrate.com/brm/amortization-calculator.asp