It may sound counterintuitive, but private equity is all about debt. Private equity firms typically borrow more than 30% of a deal’s value from banks, and then use that leverage to juice returns. And when credit dries up — as it did in 2009 — PE firms slow their investment pace, no matter how much “equity” they’ve got lying around.

But the real key of leveraged financing is that it seriously protects down-side risk. The loans are carried on a portfolio company’s balance sheet, not on the private equity firm’s balance sheet. If a default occurs, the private equity firm is only out its equity — but the debt losses are incurred by banks or whoever else has bought the loans.

So it was interesting to learn last week that Silver Lake Partners and its investment partners had “guaranteed” the debt for their $1.9 billion purchase of Skype from eBay (EBAY) in late 2009.

My initial thoughts were threefold:

1. 1. Wow, they must have been really confident.
2. 2. Maybe no bank must have been willing to lend money for this deal.
3. 3. Why do this?

1. Yes, they were really confident. This was the largest check Silver Lake had ever written, and infant investment partner Andreessen Horowitz was putting its nascent reputation on the line by pushing the limits of its “$50,000 to $50 million” investment strategy. Even if Skype’s founders had won their legal IP claims — they eventually settled, in exchange for an ownership piece — the buyers likely believed they could rewrite the necessary code.

2. The deal was announced around in the midst of a credit crunch, but it was easing.

3. This is the most salient question: Sources close to the deal tell me that the guarantee was made in order to make the debt cheaper. This obviously ties into #1 (buyer confidence), but still feels like an needlessly large risk to take. Every investment carries certain risks — as Skype itself laid out in its IPO filing — and a blowup could have caused Silver Lake’s investors to bail on subsequent support (just ask Parthenon Capital or Forstmann Little what happens when an out-sized bet goes bad). Seems like a needlessly large risk.

To be clear, I’m not suggesting that Silver Lake didn’t have the right to guarantee Skype’s debt. Most PE fund agreements include provisions that allow them to guarantee debt up to a certain percentage of fund capital. And it has happened a few times before, albeit usually on turnaround transactions where the purchase price includes virtually no equity (e.g., TPG once guaranteed debt on a deal that included just $6.00 in equity). Moreover, there are some portfolio management advantages to guaranteeing debt, such as keeping dry powder drier longer (since few funds are permitted to recycle capital

But just because you have the right to do something doesn’t make it wise.

I know that questioning any aspect of the original Skype investment seems strange in light of Microsoft’s (MSFT) agreement to buy the company for $8.5 billion. Silver Lake’s confidence has been rewarded, and its investors will never be asked to pay for a dime of the guaranteed debt.

But there is a fundamental reason why private equity usually outperforms other asset classes, and it’s that much of the risk is outsourced to Wall Street. Altering the model for the sake of a few basis points seems needlessly risky. Skype should be viewed as an outlier, not the new status quo.