Well he may be telling you the truth. There's a saying called 'Playing The Numbers'. What that means is within their real limit of bandwidth the server has, 99% of accounts aren't using anywhere near their limit. Say there is 200 accounts on a server, all small and using less than 10 meg a month. That's 200meg, or roughly 2gig. The server has 40gig set on it. So, that's 38gig to play with. If you use 30gig a month then they're still 8gig under.
No problem.
The problem is when one or more accounts goes into overload. Say 10 of those accounts install a forum, and suddenly lots of people show up. 10gig a month isn't too hard to get running an active forum. 4 accounts later using that, servers bandwidth is full. The accounts get shuffled (one to each server), to get them under the limit again. Either they get more servers, or start to pay for the extra bandwidth somewhere.
As long as *most* accounts are small and don't generate much traffic it's sustainable. Of course Murphy's Law is rife on the internet as any host will know, and playing the numbers can be a pretty big gamble. People who set limits on accounts are doing so to provide a sustainable long term business approach. People who allow unlimited space and transfer (I believe), aren't looking too far ahead and compensating for future problems. THE most expensive part of hosting is bandwidth. It's quite easy to spend more on that, than the hardware itself. The telcos and upstream suppliers make a nice profit out of bandwidth, and I can't see that changing any time soon.
At the end of the day, it's a judgement call. If you feel a deal is to your benefit, then go for it.

If there's any feeling it make come back to bite you, just make sure you have something in place to make recovery as painless as possible. Have a second choice you can move to fast, and have a regular local backup copy of your site ready to roll. It's seat of the pants action, but if saving a few bucks outweighs the stress then there's no reason not too I suppose. Were it me, I'd just find someone I could 'set and forget' the site with, feeling I'd made the right choice.
Greg Moore