Given how many elites the old UA has (just look at the number of views/posts in this forum alone compared to other airlines and even compared to CO) it would be INSANE to board all elites at the same time. I've seen this mess at CO when all elites board at the same time. I think that all elites should board using the red/blue carpet but by status level (as a policy). If there are not a lot of elites, the three+ groups can be combined into one group.
I am not even thinking about bin space here... all elites will get bin space either way, but the stampede that happens when all elites board at the same time on elite-heavy flights is crazy.
I think the zones system is more flexible. You can add and remove a zone with a certain group of people as you see fit and they won't know it, and those behind them won't know it. It's also easier on int'l flights as someone (I think fastair) has pointed out.