Re: ThunderHex inspired question
 

To add another data point, my old team used 6061 in drive shafts back in 2011 and we had several failures, most often at the center shaft between the gearbox and the power transmission. It is really not too difficult in a drivetrain to fail 6061 hex shaft and thus I really don't think there's much use for it when 7075 is so readily available.

To answer the OP's question, as to why not just use 14mm bearings with hex shaft turned to 14mm max diameter a la ThunderHex. That's a perfectly viable solution and you'll probably find success if you give it a try. The reason that regular 14mm metric ID bearings aren't used with the ThunderHex system is that these bearings aren't readily available in the FRC psuedo standard 1.125" OD for obvious reasons. I'm guessing the designers at Vex realized they had to make custom bearings anyway, and thus decided to go with a slightly more optimal but less standard size for the ThunderHex ID.