threemiles - there is no point if it is done early since as Tyro has said you merely flatten the approach angle making an accurate touchdown more difficult. If you look back, however, you will see that the same a/c the previous day had been landed 500' short of the normal touchdown zone (and only just stopped). This can only be done safely and accurately by a LATE adjustment to steepen the approach, ideally in the last 200' or so, which would then of course generate a 'low on glideslope' indication although probably not a warning (737). This was a standard technique in my day when I flew highly-swept fighters to put the beast 'on the numbers' from an instrument approach.
BOAC;
I disagree. If it is done early and continuously throughout the approach the approach angle does not change. You are merely paralleling the "on glideslope" angle. Fly the last 1000ft. exactly one dot low or one dot high and the approach angle is the same as "on" the glideslope. The only parameter that changes is the trajectory point of intersection with the runway (APPROXIMATELY the 500ft mark one dot low and 1500ft one dot high). A LATE adjustment is dangerous. It was called a "duck under" and was forbidden at my previous employers (we did short and slippery operations many times daily for decades). Fortunately most of those runways have now been extended.