Originally Posted by HTupolev

Evenness isn't all that important, but tightness can be. There are two ways in which evenness is useful...

One, erratic unevenness can mean that some of the gears are "wasted" being needlessly close together, leaving bigger gaps between other gears. The classic example of this is the conundrum of where to switch from 1-tooth jumps to 2-tooth jumps on a rear cluster; a 15-16-17 progression is tighter than anyone really cares about, but a 15-17 progression can be annoyingly wide sometimes. Throwing the 16-tooth cog between the 15 and the 17 means that gear steps elsewhere in the cluster will be wider (or the cluster will have less total range).
There's not really a perfect solution to this problem, it's a consequence of the mechanical realities of drivetrain types. In the case of derailleurs, it happens because the cogs need to have integer tooth counts.

Two, drivetrains that take advantage of interleaved ratios (like half-step) work best and are simplest when the rear cluster is as evenly-stepped as possible.