There is no such thing as a dirty bitcoin. The idea is absurd. Yes, I know what you mean, but bitcoins are fungible, trying to classify them with such things like "taint" ... ... unless you blacklist all unknown addresses, you can't implement this. If you just have to whitelist known addresses, then this function is limited.
What would be nice is if that was 100% true.
For me to send you
BTC and you to send to Alice and Alice to send to Bob that is true.
But for you to send to me and me to send to Coinbase (or Bitstamp or Gemini or other places) to convert to fiat then yeah people have been hassled about coins that were only one or two steps away from something that they did not like.
Now, I am not saying it's fair or good that it happens, but it sadly does.
Will services like this help? THAT is the discussion. You send me
BTC that I want to cash out at exchange "X" once you send it to me it's too late. Either I send it back and ask you for other coins (probably never going to happen) from another source or I deal with it.
If I at least have a heads up that you sent me coins from Roobet.com (picking on them because I am wearing their sig at the moment) and I want to convert to fiat at Coinbase (who are known to kick people for using coins from casinos) then having a quick way to check is possibly useful.
Just my view.
-Dave