The equity you have in a property is an asset that can be siezed as part of a lawsuit. The theory is that a leveraged property is a less interesting target for a lawyer than a paid off one.
Leverage is a two edged sword. If your investment gains in value, you can pocket all the gain for a lower investment. If it falls, though, you can end up losing all your investment. With leveraged stocks (i.e., buying on margin), you'll get a margin call, and be forced to pony up the losses or sell the stocks. With real estate, you can hang on hoping for a recovery until you're deep in the hole. Just ask the many, many people who are underwater on their houses.
Leverage can allow you to accumulate more properties than buying cash. It can also produce a higher ROI. It can also leave you with a loss you may struggle to repay if the situation goes against you. I don't really think there is a right or wrong answer to this question.
Pretty sure, though, having investment property that you cannot quickly sell and repay any debt is a very risky position. Realistically, I think that means you have to have at least 20% equity in any leveraged property. 10% to cover the transaction costs, and 10% to discount it for a quick sale.