well, manufacturers also have a "liability" factor involved with customers sueing over a fan that cut them because the case didn't have a grill
I chopped the heck out of my case, front, rear, HD supports, everything...It makes a huge difference and it makes it quieter.
A little mod I did recently was sand the edge of the fan blades down a little. Cutting thru the air better. A little over the top, but they seem a little quieter....i was really bored.
Here's a pic of my case before I put everything in, I chopped the rear 120 out, and the front one (behind that fan). What you see is the HD cage behind the fan, recently I chopped 2 of those bars out, so it's all open behind the fan now