portfolio diversification by definition reduces risk. Leveraging real estate investemnts changes the risk profile rather then increasing or reducing risk.
In real estate, it is diffricult to lose it all, because you are purchasing an asset that has value. However things can happen that make your real estate lose all or close to all of it's value. These catastrophic instances are rare.
By having a deversified protfolio, you become able to absorb a castrophy at a few properties without your real estate business going belly up.
Most of the time when real estate investments fail it is because of manigerial problems or market forces (high vacancy, etc...).
Market forces tend to be localized in the real estate business, by having a deversified protfolio, you essentially eliminate market risks, because people need to live and work somewhere.
If you suck at management, then you are hosed whether or not you leverage your investments.