One big question that hasn't been brought up is whether it is better to have everybody be richer with a wide income disparity or to have no income disparity? The income gap is widening in the US, but the adjusted income has gone up at all levels. It's an interesting thing to ponder on.
I do not believe that having everybody be richer and having less income disparity are incompatible goals. When you look at standard of living as opposed to simply financial ratios, then history suggests that there is a (soft) limit to how far the standards of living of the poor and the super-rich can be apart. Even poor people in the Western world today have a better life than the richest kings in the middle ages, as long as you consider basic necessities like food, sanitation, housing, education and health. (Happiness is another issue, because happiness is linked to social status and inequality.)
I am not really sure which mechanism is mainly responsible for limiting the ratio between poor and rich, but mostly three reasons come to mind:
1. At some point, the poor will stop accepting an increasing gap, and they will cause social unrest that keeps the rich down. Clearly we shouldn't let it get to that point.
2. Innovation tends to build on things that are widely available. Flushing toilets are only invented once flowing water is available to a significant part of the population. Certainly the kings of the middle ages could have been able to afford such toilets, but nobody ever came up with the idea because the idea only became natural once plumbing was widely spread. Similarly, fancy software packages would not be available even for the super-rich (to that extent) if it weren't for the fact that cheap electronics make it easy for new generations of software developers to emerge.
3. In an advanced economy, economy of scale matters. Luxury cars would not be at the level they're at if it weren't for the massive system of automobile economy behind them - but this massive economy only exists because the majority of the population can afford cars. The same thing applies to basically every technological product. The iPad would not exist even for the super-rich if it weren't for an entire ecosystem of businesses such as chip foundries that can only exist because even the poor can afford electronics.
Especially with points 2 and 3 in mind, I would say that a low income disparity is
necessary for everybody getting richer, at least for the kind of developments that are based on technological advances, which are more or less the only kinds that are left to us now that natural resources are starting to become depleted: The living standard of the majority is the foundation on which new luxuries for the rich are developed. Of course there are other necessary factors in play, but this one seems to be overlooked by most people (and the trickle-down crowd gets this wrong completely).
Slightly off-topic, but I just wanted to say that I appreciate the fact that we can have a civil discussion on such a politically charged topic.