That's how I drain my system, at idle.
Imo the risk of contamination is small.
#1Most likely a careless operator that does not clean couplers. #1 leads to #2
#2 Most likely and worst is a failing hyd component putting steel into the oil. if thats the case your loaders filter is designed to catch it but it may still cause problems. The safest bet for continuos flow attachments would be to buy a return hydraulic filter and fit it to the attachment return line to prevent any debris from coming back into the loader. After you use the attachment awhile, cut open the filter and inspect it for steel, aluminum, etc. By then you should know if the attachment performs normally or not.
I'm in the process of fitting hi pressure supply filters to my attachments so #1 does not seize up any motors on me again.
Purging the oil can help, but the problem is the attachment is cold (be sure the loader oil is warm) and may not flush all the particles out, and second you need to know why the particles are in there. As for water that is fairly unlikely and if it has water you may have serious rust contamination to go with it.
Ken