Exactly this.
There almost certainly has to be some agenda, even if it is not yet obvious.
I recently had a user that was quite convincing and made it all the way to Trust Level 2 before becoming blatant about their product promotion, having just hinted originally.
The trust level link setting is useful and dodgy accounts will start to suggest people search for xyz via google in lieu of being able to post links: another red flag.
Sometimes you just have to wait until the amount of evidence mounts up to be sufficiently sure to ban an account.
Unfortunately delaying action does mean the clean up operation is more work later on.