For me the main motive for the partial cover is protecting my pants cuffs. One pair of good slacks costs more than a chain cover, even if you buy one with the $38 shipping that kickstart is recommending so it just makes sense to do something. I probably get an aerodynamic boost with the bare metal strip ... not that it matters much with that bike.

For a fully enclosed or partially enclosed case the only rational is protecting the chain and gears as far as I know. Thankfully we don't deal with road salt here, but hard rains can drive grit and mud onto the chain so it's still an attractive goal.