I think the benchmark should be sustainability.
Offers that do not have a reliable means of sustaining themselves long term should be against the rules.
By calculation using actual data from my servers, giving away 1TB of bandwidth and 100GB of space with a twin banner would actually result in a reasonably fat bottom line assuming that the server was robust enough to tolerate it. It isn't the size of the plans that is the issue, it is the lack of profitability that causes the hosts offering them to vanish.
But if someone offered that same 1TB of bandwidth and 100GB of space as a post2host, or without any profit mechanism at all, the result is an unsustainable offer that gives clients a poor experience and makes them unlikely to come back here in the future.
Thus rather than barring unlimited itself, change the rule to block unrealistic offers where unrealistic is defined as any offer that when verified by other hosts against actual server statistics would result in little or no profit.
If need be, it would probably be possible to post benchmark traffic to profitability ratios for different advertising types for use as a standard reference.