Actually this isn't true in every case. For example, I'd rather put lumps of cash into investments that net much more than the 3% loan.
I recommend a modified version of this. Save and buy a used car for cash - a reliable junker is what you're after, or as good as you can afford. Then immediately start making "new" car payments to your savings account. When you have enough saved to pay cash for a new or late model (2 or 3 year old has suffered the bulk of new car depreciation and is a better buy) vehicle, buy it and again start making "new" car payments to your savings account. Only buy vehicles that are rated well for dependability. The obvious advantage to this system is you earn interest on your savings account and never pay car loan interest. The net interest gain between what you earn on savings and what you "don't have to pay on a car loan" is a healthy return.
TitanNV is correct, you can invest wisely and make more money than loan interest rates - but you can also lose your posterior. I've done both. Using the system as I've outlined above, I've been able to pay cash for all my vehicles since 1985. There's a not so obvious advantage when doing this, being able to pay cash most always results in being able to shop for price and get a better deal than when financing.
There's an old saying, "It takes money to make money", and that's certainly true. Initially it usually takes lots of hard work, self-restraint, and sacrifice to acquire enough money to reach the "takes money" part. After that it's easy.
Disclaimer: I'm not so Pollyannaish as to actually believe many people would embrace the idea above, really. A lot of people are into instant gratification which doesn't fit well with the plan. E.g. I had an office manager who asked for vacation and unpaid time off to go on a trip to Hawaii. I knew she was habitually broke, though I paid her well, so I causally asked how she was able to swing the vacation (rude of me, I know). Credit card!! - which meant she would be paying at least twice as much for the trip over the next few years. (When I founded the company she worked for, I didn't take a vacation for 5 years and worked most weekends. That was in 1978, memories of the struggle have fortunately faded and softened some.
