Of course, there is HF attenuation as a result. For surround movie content, however, I think that diffuse field is more important so this is a good tradeoff.
It's a matter of choosing the lesser of two evils. The best solution for ambient enveloping surround for movie content is to use dipole speakers. If that's not possible, then I think envelopment is more important than frequency response for surround speakers. Hence my recommendation.
Still the idea is a horrible one. One of the reasons why Audessey and others like it exists is to help match the speakers with each other to compensate for differences caused by room acoustics and perhaps mixed brand speakers and help make speaker transitions seamless. Pointing speakers directly at walls or ceilings will definitely spread the sound around the general area but it will sound totally different from the front and transition from front to back will be far from seamless. I have a better idea. One, move the surround speakers as far away from the listening position as possible. Two, use the distance controls on the receiver. Between those two, you will have natural seamless transition and definition in the surround channels. If you have a large area to cover, use dipoles or several direct speakers.