I don't have any data to suggest that larger companies have bigger conference calls, but it may still help the value perception of more expensive plans to limit by number of callers. In fact, most of our $9/month customers aren't Professionals that charge $80-$200 per hour, but people that would otherwise use FreeConferenceCall et al and are already stingy about spending that much. A lot of "randos" signing up from Google or ads are very small companies, non profits, organization committees for various things etc. I like those customers and don't want to lose them, but I agree that I should find a way to separate them from the lawyers that sometimes get the $9/month plan as well.
The reason you'd need multiple lines is if you're multiple people. The premise of HipDial is that each person has their own, dedicated conference line and phone number. E.g. sales teams with a few people will sign up for the Team plan.
The viral mechanics actually work out pretty well like you said. That's the reason we have the free plan, too. On the free plan we used to even send text messages to the participants after the call, asking them to use HipDial as well, but those converted so badly and gave us such a bad rep that we canceled them.