And technically you're still connecting to a soundcard - just in this case it's the built-in soundcard on the motherboard. These are usually $5 chips that are of inferior quality compared to "real" soundcards (be they external or not). Any number of adapters will theoretically introduce some noise into the signal, but a simple cable with one type of connectors at the other end and another on the other shouldn't really pose an issue.
That said, it's debatable how big a difference there is in the quality of the D/A converters of a built-in soundchip and those found in low-end purpose-built soundcards/audio interfaces - the built-in chips aren't as shabby as they were, say, a decade ago but then again you can get quality interfaces these days for not much money and the comfort and amount of control, not to mention the extra connectivity and improved latency figures you get with even a low-level external interface are worth the price of admission on their own, imo. And of course, the moment you need to connect something else than just the monitors, you'll need those extra jacks.
