The main reason for an offset kayak paddle has to do with the biomechanics of the paddlers wrist, wind resistance is secondary. Typically, most paddlers find it more comfortable to have one hand (usually the right) as a the control hand. The reason for this is that when the top hand is raised during a stroke, the top hand wants to roll back a tad. If the paddle is 0 degree, the top wrist has to roll forward which puts a good deal of stress and strain on the tendons. Short 0 degree paddles are becoming more popular with respect to freestyle kayaking, but not so much for river running.
And it has changed over time. When I started paddles were at least 45 degrees, now 30 is the most common, and indeed I have found it much more comfortable.