Any reason to not use a bag as a sleeve?

I've noticed that many of the bags are actually smaller than the corresponding-size laptop sleeve. For example, the small Wiki is 12.6" x 10" x 2" and accommodates a laptop that is 12" x 9" x 1.5" whereas the small Laptop Sleeve is 13.6" x 10.9" x 2" and accommodates a laptop that is 11.5" x 9.5" x 1.3".

So, is there any reason I should get a dedicated sleeve over a more traditional bag? I would be using it as a sleeve inside a larger bag. Would the external material of a bag such as the Wiki negatively effect using it as a sleeve?
 
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