It depends on who you are trying to protect from noise. I've visited friends by LHR, AMS & CDG. I've seen some horrendous drag in level approaches. I've watched gear down at ridiculous distances from Rwy. 160 - 4nm is great, but if you start from level 2000' it is useless to those who live at 7nm. If you make a 3.0 slope from 4000' with idle thrust it benefits an enormous number of people. If you make 3.2 slope from 2000' it benefits nobody extra. You could easily make a CDA from 6000' at LHR & everywhere else. It might be curved, who cares. It's a doddle in todays a/c. Idle thrust from 6000', 160 - 4nm, easy: less noise less fuel, job done. Tinkering with minutiae, waste of time.
In B732, no LNAV no VNAV, only DME; if you spooled up before OM you bought the beers that night. CDA from FL330. That was 1980. Come on guys, get with it! The politicians are muppets about this. Don't get sucked into it. OK, I agree, LHR/LGW/LTN/STN do make an interesting Rubics cube for ATC SID's & STAR's but if they start with the identifying the correct problem they might find the best solution. 3.2 glide slope is not a solution for anything.
I flew out of a noise sensitive airport and its ILS platform was a daft 2000'. At night they radar you to a 3000' glide slope intercept fro noise reasons. They didn't publish this, so you planned for a CDA to 2000'. If you were on a 180 trun to finals you now had to extend and make noise or use V/S. The point is they raised the platform to reduce noise, not steepen it.