Appearance and it distributes the mint evenly throughout the drink
Oh, you mean you have more control as to where the mint leaves are positioned in the glass? Does it actually have a big effect other than an added sensuary experience due to mint leaves that stick out of the drink's surface--which you could, albeit difficultly, achieve with ice cubes, too? Does the effect outweigh the reduced influence on dilution?
Yeah I don't know but you do get a bit of mint in every sip as opposed to it settling on the bottom
The dilution thing I don't think is a big deal. Large ice cubes will dilute less but it will also take longer to cool to the same temperature. It's also why you want a large ice cube for something you're going to really sit back and enjoy for awhile but crushed ice is nicer if you don't care that much and just want something cool and long to sip on