I can see from your profile that you are not a pilot. It might therefore be interesting for you to know that, although it dos not sound like much, the difference between a 3 degree glideslope and a 4 degree glideslope is quite large.
On a 4 degree glide path, you might have severe problems getting established on the glidepath if you have a heavy aircraft, a little bit of tailwind, a little high speed, and are just slightly above the glideslope when intercepting the localizer.
4 degrees is the steepest "non-standard" ILS allowed. Though I am not sure wether there are some ILS'es that are steeper somewhere in the world. (Oh yes, I guess Cape Canaveral gas a 20 degrees glideslope for the space shuttle, but that's a different story)
4 degrees gideslopes are used to obtain obstacle clearance when there is high terrain (or obstacles) in the approach sector.
Glideslopes that are less than 3 degrees requires large amounts of thrust (or power), especially when the aircraft is in landing configuration. (Gear down and full flap = lots of drag!)
This can be very critical if a twin engine aircraft flies on one engine due to an engine failure. If you (inadvertantly) get below the glide slope, combined with low speed in this situation, it might be difficult to get back on the glide slope. Some (smaller) twin engine aircraft are not able to climb or even maintain straight and level flight with one engine out and gear down. Let alone with full flaps.
Hence, a three degree glideslope gives comfortable flying both in normal and abnormal situations, for all (normal) aircraft.
Nick.
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"I have found that alcohol taken in sufficient quantity produces all the effects of drunkenness."
~ Oscar Wilde