If you're coming from something like the 350D and sports isn't your priority, then the first significant upgrade beyond the 600D is the 5D mk2. But its price is about 3x as high. So if you're budget-limited, upgrading from a rebel to another can be a very worthwhile decision. Rebels can be very far apart, especially with the introduction of video.
Well the 350D was released 2005 and the 600D 2011. Thats 6 years difference. Of course theres many things that happen in 6 years.
For me, Marsu42s post sounded like he feels under pressure to buy a new Rebel every year so he doesnt miss certain features. But i think thats not the case.
Imho i think in the future mirrorless will be the way to go. I dont see any company bringing something super revolutionary to the normal DSLR market anymore.
I think the first time u get a DSLR is the most important moment. And u can just use ANY DSLR nowadays to get good photos. The higher models dont do stuff that u cant do with a rebel too. They just do it better.
If u can wait 6 years for upgrading ur camera i think u can also save money for a full frame body in the same time