The details are that that as long as you move your funds once per year... at the expense of a normal TRX fee you will not be charged 5%.
This proves ownership, allows the network to recapture 'lost keys', and helps clean up delegate votes.
This also provides an opportunity for increased privacy because all money is moving frequently others don't know if transfers are to your self or to someone else.
The arguments above have their positive merits. This however will disadvantage silent long term holders, who might not have the technical knowledge, time, or interest to continually remember to shuffle around shares in various DACs. I can imagine that many years down the road, current Bitshares PTS and AGS holders will have shares/coins in multiple DACs, and this burden of having to remember to move them around once a year, can become onerous if you have multiple coins/shares.
5% seems somewhat high, perhaps a lower inactivity fee would balance your positive arguments, and my protection to long term holders without knowledge/time/ability to move things around.
Some (not necessarily me, but maybe) are in this for the long term, and would prefer to have a long term buy and hold without needing to move things around. The inactivity fee burdens those, and perhaps rewards those with shorter term horizons that might shuffle things faster.