Why only about 20% are forging then?
I don't know. I suspect because leased forging is a new feature and not everyone has caught on yet. Also, I suspect some whales are not forging because they want to give everyone else a chance. Either way, it should improve over time. Nxt is 6 months old; it's achieved a lot in that time, but some things mature slowly. When it's as old as Bitcoin is now, we'll have a better idea if this is working.
The downside is centralization of power. If at any point of stake history anyone had 50+% of current stake rate (i.e. current is 20% means that lending to a pool more than 10% at any point) it will haunt the coin forever.[/b]
I gather the Nxt devs have a solution for that.