GK....Let's chalk it up to the 'observer quantum effect'...or at least, it's a handy thing to blame it on...
One unconsciously feels that 'anything' between the driver(s) and your ears 'Must Be Doing Something'....
Either the air molecules are not able to 'pass through' the fabric/screen, or transmit the waveforms in the air without some resistance from said grille material in some fashion.....
Or we just prefer naked things in front of us.... ;)
ESL's aren't going to work very well without them...
f208frank's has a good rationale for being a future 'baddad' and current one for not being 'a tolerant host'.
That, and keeping the pets at bay.....
"....Dear, why is the cat in the deep-freezer?"
...embarrassing questions like that.....