There are an awful lot of newbies who plagiarize, and there are very few of them who then go on to become an asset to the forum. It is very much the exception, not the rule. Not banning someone for breaking the rules in case they then go on to do something beneficial in the future would mean that no one gets punished for anything ever. If you have become an asset to the forum, and a minor infraction from years ago is found, then some leniency seems fair, but if we offer that leniency to every brand new account then the forum would rapidly deteriorate in to an unusable spamfest.

In terms of the OP, I tend to agree. Reporting spam from over a week old is usually a waste of time since the spammers will already have been paid for it and no one is reading it anyway. It's reasonable to report more major things like linking to malware or plagiarism though. I think the place we seem to be at regarding temporary bans and signature bans if an established and beneficial user is found to have made some historical plagiarism is probably the right balance.