Chris, My airline has a policy to keep ATHR on (unless it is u/s).. and so far I found no problem with it. In my opinion it does a pretty good job, considering the G/S mini which limits the change in thrust... Yes, may be I have not scruitinized it well enough like you did...
in my knowledge, managed speed is the target IAS (=GS mini + current headwind component). it is not the minimum speed for sure. when I fly with manual ATHR (in simulator only) MAGENTA IS MY TARGET.. occasionally I may trend below it .. but thats o.k. as long I remain above the amber band.
My previous airliner did not have this feature and my experience with 'ATHR off' does not makes 'ATHR ON' any different...
One thing I hate most about airbus is the RETARD call... at times, I found it most irritating.. During my early days on airbus, I screwed up few landings, because I retarded the thrust levers listening to the RETARD callout only, rather based on the requirement...
cheers