If I could find a backpack that looked tidy when worn with a suit or sport coat, I'd use it, unfortunately, I haven't found one that doesn't ruin the line and cause wrinkles when worn. I carry a Timbuk2 Ram as my business case and I remove and stow the shoulder strap and carry it in my hand when in formal client meetings.

Also, since I don't use a wheeled carry-on, I find that a backpack really makes walking awkward with my luggage. I normally use an Ebags Motherlode convertible for my main bag and find that my briefcase provides a counterbalance when walking using either shoulder straps or handles. Also, when I have to walk a long distance (a rare event, thankfully), I put the Motherlode on my back and attach my briefcase to the rings on Motherlode's backpack straps (balancing fore and aft).

Given the more casual business environment, backpacks should be given serious consideration for those folks that operate in those environs but not everyone is lucky enough to be able to cast off corporate atire.