Its strange that so many people get offended by this and not take the joke at all, but it plays right into the nerdy engineer stereotype. Are people so uncritical of their own hobbies that they cant see how someone can find it weird or even funny? So many people want to restore cars that were not even that great for their time. They spend so much money restoring these cars and if you factor in the time they spent they could probably buy a modern, brand new supercar for the price they paid. I loved my first car and spent countless hours working on it, and had some of my best memories in it. I would never, however, spend so much money and time restoring it that i could buy a brand new Porsche instead. Even if i did, I would certainly have a sense of humour about it.
Ok, let me try to explain this one because there are a bunch of pissy guys in the back row who are feeling gratuitously insulted. I think all the joke depend of the characters personalities.
Let me try a simile: guy says "I like to restore 3600 db modems". That' s what Dilbert is hearing. The guy is boasting about putting an old device back to the montage line conditions. No improvement, no customization, just putting a great deal of time, money and effort for an assured diminished return. For his analitical mind, the hobby is clearly unproductive in every aspect, thus useless.
Wally on the other hand, is very proactive in the field of uselesness, he even took golf classes for Pete' s sake. That Wally takes interest is what drives the car home.