I just depends what you shoot. I have a 10D and it's fine, I actually enjoy working the camera to the max, learning new tricks on how to get the most out of it.
This being said, I'm upgrading soon as I want to take more night pictures and I just don't see myself obtaining the results I want with the 10D.
So yeah, nothing wrong with using an old camera to the max. It's nice, actually. But at some point if you go "well, I just can not get that shot with my equipment" repeatedly, then perhaps an upgrade is in your future.
Some people upgrade to the newest and latest as soon as it comes out, others wait for a dire need for features. In the end it doesn't matter as long as you still want to get out and take pictures.