Rich it is just simple diversification. If you have all of your properties in one market, then you are subject to any downturns in the local market that range from a simple loss of employer to major natural disasters such as earthquakes or hurricanes.
Diversification is a conservative approach, but if you have everything in one market and disaster strikes, then it can be a problem.
I am in Memphis and believe it or not this is an earthquake zone. What happens if I own 70 houses in Memphis and the New Madrid fault has a quake like happened in the 1800s?
If that happens I would rather only lose a percentage of my real estate portfolio than all of it and possibly my regular source of income as well.
Now if nothing happens in your area, then there was no need to diversify, but I can't predict the future. Hopefully everything works out in your area, but I am probably more conservative in my investment strategy.
Diversification is a huge consideration in stock/mutual fund investing, but hardly given any consideration in real estate investing. I personally hadn't given it much thought, but I did after talking to Katrina refugees who were landlords in New Orleans.