Add me to being too lazy to use USB.
I do if I need to transfer a bunch of books but for a couple I just email them to my free Kindle address.
I like that Calibre lets you email books to your Kindle but I have not figure out how to get plugboards to work with this feature so I just save to disk which is my Dropbox folder then email them to my Kindle.
If I am away from my Desktop I use my phone to download from Dropbox then email the book to my Kindle using gmail from my phone.
How's that for being lazy?