What we're up against is that providing raw numbers for usage limits leads to several problems. Among them:
1. Competition. If you provide N gigabytes of storage/bandwidth, another provider will offer N * 2, and they instantly look more competitive, even if they aren't even actually capable of providing that. So, back when we provided limited plans, we were constantly being forced to increase our resource promises to unrealistic levels just to avoid looking stingy.
2. Expectations. If you provide N gigabytes of whatever, customers will expect (and demand!) that their site be able to use up all of their provided resources. This is both on a policy level ("what do you mean I saturated the interface?") and on a technical level ("why can't I serve 100 gigabytes of dynamic HTML a month?") This becomes an increasing issue as competition drives the provides up, and the actual resources you're supposed to provide are absurdly high. (Consider for instance 100tb.com, which was mentioned earlier in this thread - good luck actually pushing 300+ MBit constantly.)
In practice, the policy we've got now actually works better in some ways for customers because they don't have to care whether they're exceeding resource limits, either subjective OR objective. So long as they're running a site which complies with our Terms of Service, we'll do our best to keep it going, even if it gets big.