I've seen this exact thread rehashed numerous times over the years. I understand why it's irritating to receive marked coins, especially if it's a coin you are interest in keeping. I wouldn't directly encourage anyone to do it. That said....
Probably 20% of the people reading this thread have marked coins. I think everyone is just staying quiet to avoid the name calling

I dated a bunch of coins back in the day. It helped me determine certain things...it was very helpful. If you stated it won't help...you're just wrong. It may not help help you, depending on the circumstance and knowledge of your courier/coin services...but it helped me. Not all regions are handled the same, in terms of courier service and whether coins are re-rolled by a bank, a courier, or returned to the fed. Determining the appropriate bank to buy and dump, be it a local, regional, or national bank can make a huge difference in your success rate.
Yup, it has been hashed and rehashed. What people do not understand is that the only feedback/data that ever matters is how productive your system is. There are scenarios where dumping into your source stream improves your productivity. If you follow the logic, the only possible decisions you could make after marking a bunch of coins are the same decisions you should make without marking.
1) You are getting good productivity and not finding any of your marked coins -> continue with current system (but increase your volume)
2) You are getting good productivity and finding some/few of your marked coins -> continue with current system (but increase your volume)
3) You are getting good productivity and finding tons of your marked coins -> continue with current system (but increase your volume)
Sure, you could dump somewhere else, but it might very well be your dumps that are stirring the waters and pushing keepers your way. Don't fvck with a working machine.
4) You are getting lousy productivity and not finding any of your marked coins -> scale back current system and change it up with other sourcing strategies
It would be stupid to stick with a crummy system just because you know you are not getting your dumps.
5) You are getting lousy productivity and finding some/few of your marked coins -> scale back current system and change it up with other sourcing strategies
6) You are getting lousy productivity and finding tons of your marked coins -> scale back current system and change it up with other sourcing strategies
You could change to a different dump scheme, but that will just put you in scenario #5 which has the same corrective action as this scenario.
The presence or absence of your dumps is meaningless all the way around. You should always be attempting to increase/vary your supply side to find the most productive solution. When a particular supply strategy is productive, exploit it. Otherwise, find an alternative supply.