Well, you figure on the 99-01 models (I have a 2000), the hole is plugged by a sensor. For the 02-04 years, that sensor was included with the MAF to simplify things, but this leaves that sensor hole open. Leaving a hole past the MAF is always a bad idea because the engine will pull the air in the fastest way possible, and when this happens the MAF cannot meter the air that comes in through the hole because the MAF is before it. If the MAF cannot meter the air, the ECU will not put enough fuel into the chambers leaving you running lean, and since the MAF and O2 sensors will show different readings, it will only throw a code and make the engine run bad. Too lean causes extreme heat inside the combustion chambers and can lead to pre-detonation. The heat can melt pistons and pre-detonation can cause unwanted stress on the rods, eventually possibly causing failure. MAC, BBK and K&N all include a gromet to cover this hole for 02-04 years or they do not have the hole at all.