From a physics standpoint closing the racquet face will create topspin relative to more open racquet faces. That's not a debatable point, it's just a fact. The thing is closing the racquet face will also change the launch angle of the ball all other things being equal. That also isn't a debatable point.
How much topspin does closing the face contribute? I don't know, but as a player I really don't care either. I have to get the ball over the net and inside the baseline. If all other things in my swing remain constant, then racquet angle is the only tool I have to make that adjustment. I'm not going to think, "Wow, I want more topspin, I'll close the face." because I'll just drive the ball into the net. If I want more topspin I'll increase the vertical component of my swing. For a specific swing I'll adjust the racquet face to get the right distance over the net and inside the baseline.
The racquet face angle has to account for the upward component of the swing (the more I swing up the more I need to close the face), the incoming trajectory of the ball (if I hit on the rise I have to close the face more), the incoming spin on the ball (the bounce takes away a lot of the initial spin, but a good slice will still have a lot less topspin than a topspin hit - the sliced ball needs a more open racquet face), how hard I'm going to hit the ball (close the face for a harder hit), and where I am on court (the further back I am the more I need to open the face to get the depth). With all of this going on, in the space of a few hundred milliseconds, I need to keep it as simple as possible. Swing path for more or less topspin, racquet angle for launch angle.
I agree with Cheetah that if I hit harder I have to close the face, but I just think of it as a necessity of physics (see previous paragraph). Softer hits can be (should be) higher over the net. Harder hits have to go lower.