Here is one reason for that lil' ol' plug. Take a late 80's Chevy pickup with a TBI. Those TBI units have a tamper proof plug for the idle set. Easily drilled and then the idle can be adjusted. Let's say your idle is a bit high. Makes sense to drill the plug and lower your idle, yes? Not necessarily so. Idle (amongst other functions) is controlled by the computer. Let's say one of the TBI flange gaskets has developed a leak. Your idle will go up. Instead of repairing the problem, the vacuum leak, you have changed the factory idle setiing. Sure, your idle is good, but your truck is not fixed. Pretty soon you develope driveability problems that can't be corrected by adjusting that idle screw. You find the leak, repair it and now you can't pass smog or get the truck to idle right 'cause it takes a scope and other stuff to get the idle back into the ballpark.
First thing I did when I got my 1990 GMC R3500 was replace the TBI with an unmodified version. After that, the rest of the problems were easy to find and repair.
