It's like this - 30 yr fixes are set up where you pay more interest then principal for around the first 15 years. So your average homeowner does not keep this loan for more then 10 years at the most (yes some do keep the loan for the full thirty years but they are a very small percentage). Your average homeowner will refinance for different reasons, pull out cash, lower the rate/payment, sell the house , etc... So if you get a 30 year fixed and only keep it before the time you really start paying off more principal then interest, then all you've done is give the bank money. 30 yr fixes have higher rater then adjustables , so for that time you're paying the mortgage, you're paying a higher payment yet not really paying off the principal but the interest. Adjustables get a bad name cause people actually think borrowers actually let the rate go adjustable. You don't let your adjustable to adjust, you just refi into another adjustable (unless you fuck up your credit and/or you have no equity).
A good way to think about why not to pay off principal is cause it makes no money for you. If your property makes money by market conditions, it really doesn't matter how much money you pay off in your house. The money you put into doesn't make money for you, cause it rises in value by market conditions, not how much you paid off. That money paid off is stagnant - yes its safe but it makes no money for you and how are you gonna get to it if you really need it? Sell the property ? what if you don't want to sell the property. Refi? well if youre credit is fucked, someones gonna jack you on rate and closing costs in order for you to get to that money. Might as well pay a lower rate, keep the money in a saving account and be liquid.