About 2 years ago I too had added a 5D mk1 to my 50D to see if I wanted to go to full frame. Gradually, I started using the 5D all the time, even after I picked up a 7D 6 months later. Last March, I decided to sell both the 7D, 5D and my Canon 70-200 f4 to get the 6D and a Tamron 70-200. I do wish I had a backup camera at times, and will probably get one this spring, but overall I am very happy with my decision.