Electric motors have areas of operation where they are less than their peak efficiency, not unlike ICE's. The solution, modify the gear ratios in-flight (efficiently), of course. Tesla wanted to start with a two speed but didn't get the bugs worked out at those power levels I recon. The simplicity of direct drive is nice, but leaves a significant amount of efficiency on the table.
They claim 15% reduction in battery size (which translates into less weight and shorter recharge times)
Drive System Design: New three-speed transmission helps electric vehicles run farther, faster and more smoothly » Automotive World