Have you heard the 200s in surround and rear orientations?
I'm sure the Sierra 2s or the new Sierra sats would be closer to the ideal- but even those aren't ideal, only closer to it. Anything less than identical speakers all the way around is a compromise of some sort.
So since you are going to make compromises right the bat in the most critical area ( the front three) as most people are, why then avoid making a compromise in the least critical area?
Just like, after finally having heard it, I don't think the Horizon center is a compromise from three S2s as L/C/R, I don't feel like the 200s are any compromise as rear surrounds.
They're not only the most economical Ascend choice, they also just sound pretty damn great to my ears in those locations.
In fact, I've posted this before but it made a strong enough impression that I can't help but repeat it- The 200s sound more impressive as rears for the RAAL equipped fronts than the sounded for the closer related 170s. I don't understand it, but that's what I'm hearing.
If you already own the S2's and can swing the rear port in your placement, the savings over switching to something else might not be worth the time and trouble. But as rears especially, your are firmly entrenched in the area of diminishing returns.
JMO. YMMV, of course.