i started off as a commuter with a rack and trunk bag, but i eventually switched to a back pack for a variety of reasons:
1. aesthetic purity - none of my bikes need to have a cumbersome rack attached to them. this is especially important for my titanium road bike.
2. i'm gonna sweat no matter what - my daily commute is 14.5 miles each way; there's no casual cruising at 10mph for me, i like to hit it hard, so i'm dripping with sweat when i get to work whether i'm wearing a back pack or not.
3. handling - i prefer having the extra weight on me as opposed to on the bike. it just feels more natural to me to have the weight on my back, it makes the bike feel zippier and more maneuverable, especially when out of the saddle.
4. comfort - i tried a lot of packs, but eventually settled on an REI pack with multitudes of micro-adjustment straps that allow me to cinch the pack down in all the right places to get that perfect feel where the pack simply becomes an extension of my body rather than something i'm wearing. waist and sternum straps are absolutely essential in my opinion for a bike commuting backpack.
5. the backpack always follows me - when i get off my bike, my backpack is already attached to me, so it goes wherever i go without me ever having to think about it.
6. psychology - in my mind, i feel faster/less draggy with a pack on my back than elsewhere on my bike. that notion might not have much basis in reality (if any at all), but it's still there in my head and hard to ignore.
7. simplicity - i try to travel as light as possible. all of my bikes have a small saddle bag for a spare tube, levers, multi-tool and CO2 pump, so the only things that i'm actually carrying with me in my backpack on my daily commute are a change of clothes, lunch, wallet, phone, and keys. that's it. i don't usually carry a lock because i store my bike inside my office. and i leave my work shoes at the office so that i'm not needlessly shuttling them back and forth everyday.
with all that said, it all boils down to personal preference. there's no such thing as a one size fits all answer to the eternal question of rack or backpack. different commutes, preferences, situations, riding styles, and a multitude of other variables will eventually inform each individual's decision as to what works best for them.