Why so much leverage?

[Ed O.]

A theme I heard several times at the conference was the notion that some very successful landlords did not ever wish to pay off their mortgages on their rental properties.
Rich Weese also touched on his strategy of buying 20 homes in 20 years and utilizing leverage as well.

What are the major reasons as to why this is a favored approach?
Is it to buy as many properties as possible?
Is it to reduce income taxes?
Is it to limit liability by having mortgages attached to every rental?

Michael Zuber also took a strong position on this as well.

To any investors out there with this approach, can you offer more light on why you take this position. Also, I'd suspect investors in this position would be likely to carry massive reserves, as bad months aren't as easily offset with cashflow from units that may otherwise be paid off.

Thanks in advance!