Basicly, to use all the benefits of this highly sophisticated, excellent AIRBUS you final aime should be, trying to fly managed as long as possible and as long as practicable, means if traffic situation permits. This is the way this aircraft is designed for !
Well if you're flying into a sleepy little Greek airfield at night with no other traffic and procedural approaches then by all means use managed descent because there's nothing to disrupt the plan. If you're flying into major European hubs where the approach procedures are merely a guide and you can expect frequent radar vectors, speed control and direct routings from multiple ATC sectors then forget about it because the FMGC won't handle it. And if you try to reduce to green dot speed when the FMGC tells you at FRA you'll get an almighty bo**ocking from ATC too!
One other point is that if you let managed descent fly the profile it can level off at the platform altitude, put a fistful of power on as the speed target is 250kts then take it all off when it overflies the decel point and the target jumps to green dot. Thats a rather poor performance in noise abatement terms and also screws up any attempts at a continuous descent approach which is requested at airports like LHR.
Pakeha-boy: It's not your fault, I guess you're just not used to busy airspace down in NZ. By the way I flew the 319, 320 and the 321 on sector lengths between 150nm and 1600nm in both European and Russian (metric) airspace.