Hpbleed,
I know of instances of reducing mach to increase RoC that have ended in low speed excursions due to ALT*. Don't be too confident that OP CLB and CLB will completely save you. If setting V/S 500-1000ft/min results in a slow speed excursion then maybe the crew are not really cut out for their job.
As I said before your/our aircraft are fairly good climbers (both power and FMGS accuracy) and I have not had to use this technique at our company and I have no desire to. But there are older Airbus (old FMGS standard/underpowered) where this is a very useful technique. If you don't believe me ask any of your Captains who have flown older charter heavyweight A320/1s.
Don't even get me started with old heavy A340s where you sometimes had to descend to get the speed back to complete the climb.
It sorts it self out in managed climb and so what if it takes a while or it goes negative for a short while??
Pizza Express
Don't forget to tell ATC that you are not maintaining 500fpm as per the UK AIP.