Question comes a lot of people tend to debate do you sacrifice realism for good game play. If something is cool or fun, should you use it even if it is in the realm of absurdity. Obviously no one wants a boring game, but the debate can get a little too extreme in opinion. You can have good gameplay that is realistic just as unrealistic gameplay can be terrible.
However as I said, I don't think it is a bipolar extreme. Also focusing on one more than the other can lead to bad game design (too realistic with no concern to gameplay can lead to a boring game, while focusing only on gameplay and not trying to give it a sense of realism can lead to a ' silly fun game' that is not rooted in any reality).
When it comes to RTS I think you do need a certain attachement to realism, especially if it is set in modern times or near future on a mirror of our world.
Actually Realism is rather a loose term. Historically or politically (which can apply to both a historical or futuristic game) realistic, physical realism, tactically realistic, culturally realistic, and the list goes on. Even just in the gameplay department: tactical models, units, and just the physics each can have their own level of realism.