I have to say I'm really surprised by the number of people saying they would buy a house outright. I think a lot of people mistakenly think that they need to have as much equity built up in their house as possible. I still think it makes more sense, in the long term, to take the extra money and invest it in stocks, or to use the equity from your home and buy investment properties. With stocks you can realize compounded interest, and when you own multiple properties you can see much greater home appreciation (obviously).
I pulled this scenario from another site because I think it gives a pretty good explanation of why to only put 20% down:
Scenario #1
You buy a $200,000 home with cash. Over the first year the value of the home increases by 5%. Your $200,000 investment is now worth $210,000 which shows an increase of 5% on your investment.
Scenario #2
You buy a $200,000 home with 20% down and mortgage the rest. Your 20% downpayment comes out to $40,000. Over the first year the value of the home increases by 5%. This is an increase of $10,000, the same as in the first scenario. The difference however, is that it only took $40,000 out of your pocket to make that $10,000 increase, vs. the $200,000 out of pocket in the first scenario. That represents a 25% increase on your investment ($10,000/$40,000) instead of just the 5% increase in scenario #1 ($10,000/$200,000). Of course, you will have to factor in the interest that you are paying on your mortgage which will decrease that percentage slightly, but you still come out with a much better return on your investment doing scenario #2. In the meantime, you can take the $160,000 that you didn't spend on buying the house outright and invest that in stocks, bonds, money markets, etc.