It has been proven time and time again that you can achieve a rather high ranking off of solely your clay prowess, and winning the occasional matches on hardcourts. Grass doesn't have the ability to pad a ranking in the same way - the clay players who deserve the seeding have the seeding, and we find that those who don't still tend to do well, if good enough.
Monaco/Tsonga is a good example of that tomorrow.
In any case, if RG seeded like Wimbledon, Roddick and Blake would still be seeded, because they only take seeded players and rearrange them with a formula.
That's too general though. To say "grass doesn't pad the rankings" because it can contribute only 2750 to one's ranking (that alone would currently make someone #13 at RG) is just fogging over the issue.
I think adding some clay, hardcourt, grass, and possibly indoor factor is appropriate. Being the smallest, grass and indoor need the largest corrections. Then clay. Then hard.
But then there is the argument of "how different is indoor and hard, or what about hard variations and such?" I don't want to think about that.