All The Money On One Currency, House?

[GeorgeF]

Reading the posts about investing all your money in one stock raised a question in my mind. Diversification in 'paper' assets like stocks/bonds is generally accepted, and even required by some laws governing pensions and such. Conservative investors are not encouraged to diversify away from their national currency or their local real estate market.

1) I have never heard anyone (TV pundits) encourage America's 'retires' to put some of their savings in a currency other than US$. (although inflation protected bonds are perhaps a similar thought)

2) Why is investing 100% of your wealth (down payement) and a minimum of 25% of your future income (mortgage payments) in a single piece of real estate a good idea?