FlexPlan allows for customers who need it to "Flex" out of our standard Drupal hosting limits.
Example 1:
Customer #1 is on the Standard Plan. Her domains don't get very much traffic, and use little resources. She is no where near the 1500MB disk limit - actually, she only used 200MB! She sends us a mail and says, hey, you know, I would like to add a couple more domains, but I don't think I need a Plus account.
In this situation, we take into account the low CPU and disk usage, and give the customer a couple more domains. We realize she isn't abusing or using a disproportionate amount of resources. Why should she pay more? She will probably need another MySQL database or two, but since here current databases are fairly small, we just add those to her account.
Example 2:
Customer #2 has a Standard Plan. All domains are in use, plus a couple sub-domains running forums on their cPanel server which are getting a little popular. They use a lot of disk space, more than 80% of the customers in their plan bracket. Alternatively, it could be they use more CPU than most others. They send us a note, "hey, can I get a couple more domains?" -- We're likely to say, umm, well, you should consider buying a Plus account.
Isn't that fair to our other customers?
Example 3:
Customer #3 has a Plus Plan. They're not using all their domains. They mainly signed up for this plan because they have one website (sure, it has a couple aliases like ".org" and ".com" pointing to one domain, and a couple other smaller sites) which gets a moderate amount of traffic. Still, they're not heavily used. The customer sends us a message saying "Hey, I could use some more Mailing Lists for my site" and we look at it. Okay, why not - there's no reason to make the customer upgrade to a Super for this.