I added the spoiler for mine because I've always like car with lip spoiler and I'm sure it won't make the car look bad. Plus I don't want to deal with aftermarket spoiler/color matching.
I think it gives it that extra sports look I want one. I'm betting as with most cars you will be able to go on ebay and find them about half or two thirds the price of $300.
The replica of the OEM-accessory will likely be on ebay for $100'ish shipped (and come painted w/ mounting hardware). The $300 stand-alone option price isn't that bad as it includes installation & OEM-level of quality & fitment.
I wonder if these are just affixed w/ VHB adhesive mounting-tape or there's also drilling/clips into the trunklid...
The deck lid already has a slight upwards flair at the rear lip. I don't think that the extra spoiler is going to add much if the photos are accurate. I think that MB should have made the rear deck spoiler standard on the CLA45 model to go with the eyebrow air dam treatment on the front of the car. Another subtle touch to help distinguish the base CLA250 from the CLA45.
I am a big fan of rear spoiler. However, I think it looks weird on the CLA. They don't seem to go well together. Thus, I am ordering my CLA without it. May be it will look good in person.