The first job I ever had was working in a grocery store, and you could often find me in the parking lot trying to see how many carts I could wrangle at once. That's kind of how Link Mugs work.
Back then, the danger was smashing into cars with a 50-cart runaway train. With Link Mugs it's carrying too many cups and dropping them on others in a shower of ceramic and scalding hot fluid. Still, if you are sensible about it, these mugs might be helpful if you are unwilling to spring for a tray. These mugs were first introduced as a concept a few years ago, but you can actually purchase a set now for around $53. [Mocha via Holycool via Gearfuse]