What, exactly, is wrong with doing that? I'm not averse to using VS when appropriate (i.e. to comply with RVSM requirements) but otherwise why not use all of the tools that Monsieur Airboos gives us?
Well, to get best use of the managed descent with a constraint you'd need to descend at or within a few miles or so of the white arrow. ATC may want you to go down now - in managed with a constraint, making a decent early we will have what is effectively a commanded VS as required by ICAO standard minimum rates (unless within dest elevation +5000). You'll also probably end out rocketing towards your cleared level in an idle descent, possibly within RVSM.
This could be extremely butt clenching if the winds uplinked or inputted to the FMGC are not actual winds. VS and mental arithmetic always work.
Also if you are beginning descent outside 200NM - it will not perform a PERF DESCENT - it'll perform a PERF CRUISE descent or STEP DESCENT which means it plans on a VS of 1000 FPM anyway and I don't believe it's use considers restrictions.
Then there's the way Airbus have decided to programme managed descent profiles - idle to the first constraint and geometric to the next constraints if cleared below the next constraint. Personally I like to perform a DIR TO after each constraint to reset to an idle descent to the next - it's more fuel efficient and typically faster on the basis of maintaining a higher TAS - unless wind below is less.
There are many ways of flying the Airbus - learn that and you'll be a long way towards good CRM and a pleasant day out.