I have to take back my original statement. It is being done on high capacity ships.
I do agree that if you are a full-suite or Platinum member you should be able to board at anytime after boarding starts.
I have now got to the point where I am not there at 11:00 a.m. to start boarding the ship. I wait and check out of my hotel room at noon or even request a 1:00 p.m. late check out and board at my leisure. I found boarding around 2:30 to be pretty easy and shorter lines. This was true on Carnival Liberty in Miami (June '11) and Carnival Pride in Los Angeles (August '10). It was nice not to fight the crowds. Lug the carry-on around for a couple of hours until the room was ready. On my other cruises during that time we've gone with groups who want to be on right away.
I don't know if I would have a problem with the staggered boarding.