as an amateur you don't need a second body but i can think of times when its worth having one the two lenses thing for a start - also still having one if one is in for repair, has a dirty sensor or whatever, plus the ability to use a cheaper body as a sacrificial one if you are going somewhere dirty or dodgy, or need to leave a trap camera out overnight or whatever
Personally my cameras get hammered so they arent usually worth much by the time i come to change them (or looking at it another way i only change them if i really need to - not simply to get the next widget) so I tend to keep them til they're dead
I currently have a 20D , a 450D, a 300D , three compacts in the digital range , and an Eos 3, eos 5, eos 30E, mamiya 645, and Holga in film (the 300D as been up on a kite, the compacts been strapped to a remote control heli etc ) I used to have 2 10Ds as well but i killed one of them while shooting gannets from a rhib, and the other one just died of overuse (shutter failure at arround 100,000 actuations)
(Thats for hobby/unpaid toggery - I do do weddings - but currently I'm hiring a 5D2 for those jobs and taking the 450D as a back up - In the fullness of time I Intend to buy a 5D2 and a 7D when i can afford it)