I took ~4BTC from these keys. What do you mean by "useless"? Useless for you? 
You did? Please give us proof and share the knowledge of how you did it. Aren't you able to walk through the dust addresses and check for withdrawings from them?
Maybe, i would suggest setting up an own block explorer locally:
https://github.com/bitpay/insight
Or better, use addrindex version of bitcoin core which gives the searchrawtransactions API: https://github.com/btcdrak/bitcoin/tree/addrindex-0.11
add txindex=1 and addrindex=1 to bitcoin.conf and start once with -reindex
With this patch you can search any arbitrary address without having to import the key.