I don't think spamming in the forum is intentional. Some do spam out of their awareness that they are already spamming. Although most of those who own multi account may resort into spamming, but I mostly believe that those newbies in the forum are most likely to do spamming since they keep on flooding post with no quality content.
All spamming are intentional, you can't wake you and start writing rubbish without been intentional. Just as the OP said, many spams are as an outcome of multiple accounts. Back in the day it was because they wanted to farm as many account as they can to increase their bounty payouts but since bounty aren't paying as they use to do, the attention has been shifted to signature campaigns.
They mightn't be able to get into the high paying campaign but having multiple accounts in smaller paying campaign with lesser requirements giive them the same pay as those with one account in a high paying campaign. Not only those with multiple accounts are spamming though but also those with one account. Here's a secret, if you are contributing to a discussed that you know nothing about but only writing there to complete your weekly quota, you're spamming. Off topic is a spam, generic replies are somehow spams.
Provided you're not adding anything new to the discussion you're spamming so when you see a threads with multiple pages there's no need writing there again because irrespective of what you feel hasn't been said must have been discuss already before the thread got to that number of pages.