I was going to say the same thing if you didn't. I will swiftly address the major arguments for backpacks over saddle/tank/tail bags.
Argument 1) It's cooler to have a backpack than any baggage on your slick bike.
Response: Wrong, backpacks almost always look less cool than sans-backpack on anyone because mostly kids wear them. If you have at least one decent bag somewhere on your bike you can use it for transporting stuff and just make sure the bag solution is quick release or removable for when you want to go hot doggin'. I think a minimal bag on the bike somewhere is way cooler than a backpack which just looks amateur.
Argument 2) Backpacks provide additional safety protection.
Response: They can also f**k up your spine if you land on them wrong. They will always make you less aerodynamic and generally increase wind drag on your body which will tire you out more on longer rides.
Argument 3) Wearing backpacks on your bike regularly will not lead to chronic shoulder and neck pain.
Response: My chiropractor would disagree.
EPB's argument does make sense though. If you've got a variety of bikes it would be tough to find something that works for all of them. Maybe a versatile tank bag would work. But having a bunch of different bikes well... that's an uncommon and good problem to have.