I think the subject prefix is useful for mobile notifications. When an email comes in, it's easier to understand "this is a message from dw-user, so despite how alarming the first few words appear, it's not something I need to dig into straight away." Otherwise I get something like the image attached and have a difficult time distinguishing it from everything else. Of course, the subject prefix will eat into the displayable characters in the notifications. So it cuts both ways.
I think the setting in Google Groups also only applies to email notifications. I don't think the subject prefix appears in the web interface, but I could be wrong.
Ryan