I don't have the facts (regs, AIM, opspecs, et. al.) memorized or in front of me.
However, here's my take on dipping below the ILS glidepath.
1) Will it cause you to go outside the parameters of a stablized approach?
2) Will the glideslope cause you to land outside the touchdown zone or place the aircraft in a precarious position? (calcapt's Quito example is perfect.)
3) Where is the performance calculated from? Is there a correction from the glideslope included? FAR 121 landing performance is a very conservative calculation.
4) How far behind and below the flight deck are the wheels closest to the dirt? For example, reaching way back to my tprop time, I remember flying the approach nose low. Obviously the nose gear was the closest to the ground. On the E145, (if I remember correctly) the deck angle was about 0 or 1 degree nose up, placing the mains just a little lower. The E170, the first jet with slats, flew a normal approach at about a 2.5 degree nose up attitude, placing the mains well below the nosegear. And, of course, now, the whale is the extreme example. While maintaining 2.5 degree nose up on a 3.0 degree glidepath, the trucks are about 50' below and about 95 or 100' aft of the flight deck. I think it was jtrain that pointed out the threshold crossing height, which was most excellent. In a typical day, do you need to be less than that?
Anyway, that's my take on it...