Logitech Speakers: Why is the Z-5500 Digital so Expensive?

Because the X-540 sounds like crap by comparison?

(Similarly, the Z-5500 sounds like crap compared to a decent midrange home theater surround setup, which will sound like crap compared to a full set of B&W 802Ds with a pair of SVS PB13 Ultra subs)

A more complete answer: the Z-5500 will play a larger frequency range (both deeper and higher) with less distortion (so it will sound clearer and more accurate), and it will do so up to higher volumes than the cheaper X-5500 will. Basically, it will sound clearer, the bass will be deeper and harder hitting, and the entire setup will go quite a bit louder without sounding terrible. As a somewhat extreme example of this, I have an SVS 16-46 PC-plus sub ($900, now OOP), a Denon 2808 amplifier ($900) and a pair of Bowers and Wilkins 685 speakers ($700) hooked to my computer, and I don't regret it at all. They are just so much clearer and more capable than any computer speakers I have ever heard (and many home theater setups too, for that matter), and I spend a lot of time at my computer playing games, listening to music, and watching movies, all of which involves the sound system. In many ways, I'd prefer to have an awesome sound system and monitor than the absolute latest computer components, since speakers and a monitor will last you a lot longer and you'll get more use out of them than out of a video card for example.

Quite a bit longer than that even. A good HT/stereo setup can last decades. My dad still has the speakers he got in college, and they still sound darn good.