I've had my 2592 CP Blissy and my 3081 CP Rhydon in the same Gym for over a month and a half now! I call them my gate keepers. My rivals where I live are intentionally leaving them in these gyms because they don't want me to power them up any more and use them to gain territory for my team. Personally I think it a poor tactic to hold my best Pokemon in gyms. I've hatched two Chansey's while my Bliss has been in a gym and I walked my Snorlax into another gatekeeper at 3099 CP. Soon as my Blissy is nocked out of the gym I will be able to max it out.
In my rivals defense their tactic to intentinally hold my Blissy and Rhydon hostage was strategic. I had been dropping these two Pokemon in the heart of enemy territory for my fellow team mates as a beacon of hope every day I could on my lunch break. The beacon worked, my team holds two gyms for a day or two behind enemy lines while I collect my coins from my gatekeepers and enjoy my lunch.
My rivals have gone to great lengths to push me out of their territories but I average 5-7 Mon's in a gym a day.