It's not about POS vs POW. Nem can still be 100% POW. The mining mechanism can  be used to distribute 50% of Nem. Mining mechanism can be anything and could have no relation to Nem internal POS protocol. The purpose would be to distribution Nem --  much wider distribution and attract new audience (the mining community).  

Everyone will benefit for wider distribution. 100% of coins controlled by 3000 accounts (80% of the sockets) will do more damage than good.  

Make it 50% to stakeholders
and distribute 50% by some kind of mining competition