Some people prefer a faction due to the political statement they associate with playing that faction.
Some people prefer a faction due to the gameplay of that faction.
Some people prefer a faction because it's overpowered and easy for them to win.
Forcing them to play a certain faction from time to time or decreasing the rewards otherwise sounds like a bad idea to me. People should be allowed to play games in whatever way they want as long as it's in a fair way, meaning no bug abuse.
I agree 100%.
There may be some faction design issues with Soviets and US Forces. If so, they should be addressed. There may be some under/overperforming units for Allies. They should be addressed. But RTS games always have factions that are played more, irrespective of balance. Look at StarCraft - Zerg are played least, and while I am not up on current balance trends in that game, the last I heard they were doing pretty well.