Normally I'd agree - but on 3yr paper it doesnt have the duration for a longer term investment, and unless MSFT is buying junk bonds with it I really dont see the point for them. Perhaps an acquisition on the horizon - or maybe they are buying some CMOs
The vast majority of MSFT's cash is overseas and thus to get around paying taxes to repatriate it they chose to issue debt at insanely attractive levels. MSFT will likely use the cash to pay its dividend and buy-back shares.