Whatever AMD tries to say and do, the PR rating's intent is to give them numbers that users can compare to a P4 so that they don't feel like they're getting a "slow" processor compared to an Intel system. Therefore their rating comparisons to the Tbird were designed to give them PR numbers that would be about the same as a higher clock speed P4, for marketing purposes only. The PR system wasn't re-introduced until the P4 came out; previous to that, the Athlon was competing favorably with the P3 based on clock speed alone (lower price, higher performance per clock). If the P4 didn't have much higher clock speeds with lower performance per clock, the PR system wouldn't have been used.
While this system has obviously worked in some ways, such as magnum375 buying based on the 2500+ rating, it has also failed in some ways, as evidenced by the apparently severe upset at discovering the true clock speed (which makes the user feel somewhat gyped by AMD and the VAR), and the fact that very few if any OEMs or component dealers give just the PR number without the real speed.
Based on the P4, no, because they can't do that without looking stupid. So they "base" it on a Tbird, in order to come up with numbers comparable to a P4. No one honestly can say they believe the numbers have anything to do with the performance of a 3 generation old core that most consumers don't even know about and that isn't produced anymore. How many people look at a 2500+ rating and go "gee, that's a whole lot better value than my old t-bird!"?
At first the PR numbers were at least mostly valid, but no longer. AMD combines too many factors -- clock speed, bus speed, cache size -- to come up with the numbers to compare with P4's that are increasing by actual performance levels, not just a percentage rating. There isn't a consistent enough (meaning across the board) increase in performance to justify the wider gap between actual freqency and performance rating with the newer Athlons, when the performance rating was already starting to struggle to keep up by the time Barton was released. It's obvious that AMD simply can't get the Athlon's clock speed up to what it needs to be, so they fudge the numbers where they can, pretend not to be trying to compete with Intel's numbers, and keep making money on the fact that they're usually cheaper by a small amount and people hate Intel. I'll almost be surprised if AMD stays in business long enough for us to see affordable Athlon64 systems.