Why should you keep at least 2 maxed sets? The rule has its reasons.
1. You want to help the regiment win
The winning mentality is so vital to regiment success. Be proud of your maxed units, and show the enemy no mercy. As PatB would say, "Fight, fight, fight!" Council and regimates will recognize you for your excellent work.
2. You can only level 3 sets or less effectively each battle
After level 20, units won't be fooled by cyclers. You need to play effectively to level effectively. Each set needs about 3-4 minutes if you are playing to level (that may be why we have so few sitters, we need more unselfish sitters. thank your sitters every battle). If you are trying to level 5 sets, you have a fragger mentality and you will not be effective in battle. Don't expect any hugs or kisses from me if you are a XP-loving fragger.
3. I'm not saying you shouldn't level past 50.
It's important to have fun and test other units. Silence is a big proponent for this. Think of it this way: having fun means leveling some and winning some. Doing both effectively means using your max and leveling sets correctly.
Keep your maxed units, use your maxed units, be proud of your maxed units.
GO TOFU!!!