Ah, a question I can really get my teeth into.
First of all, let me say that the term 'backpacker' is mis-applied by all you young-uns. OK, it has come to be pretty universally accepted as the term to describe some kid schlepping around Europe or SEA with a backpack on their back but that is like calling a burger flipper at MickeyD's a chef.
For a fairly good description of the origin and history of backpacks Wiki is useful. http://en.wikipedia.org/wiki/Backpack The name 'backpack' was coined around 1910 with internal frame packs (most common today) being invented by Greg Lowe in 1967. So it is fair to say that the modern backpack most are familiar with today began around then.
The point to note is that backpacks were invented and continue to be designed for wilderness travel. The vast majority of backpacks sold are not specifically designed for what most here on the TT refer to as 'backpacking'. The exception would be the less common 'travel packs' which have a zippered flap to conceal the harness and convert them to more of a soft sided suitcase. They are designed for schleppers going to Europe or SEA. As for packs with wheels don't even go there.
So a TRUE backpacker is someone who goes out into the wilderness with all the necessities they require carried on their back. That generally means a stove, sleeping bag, perhaps a tent, food, clothing, water, etc. Not a cellphone, ipad, laptop or little black dress in sight. A backpack is designed to carry all these necessities on your back, all day, every day in as much comfort as possible.
A TRUE backpacker knows that the definition of the ultimate backpacker is, 's/he who travels with the least weight and the most comfort'. A backpacker weighs every item that goes into his/her pack to try and achieve that ultimate goal. It is not uncommon for example for a backpacker to cut off half the handle of a toothbrush and even drill holes in the remaining half, to reduce weight. This semi-obsession with weight cannot be stressed enough and is a clear differentiator betweeen a TRUE backpacker and a wannabe schlepper.
A TRUE backpacker doesn't use roads, rail passes or have pre-booked accommodation. All that's needed is a topographical map and a compass. They are the ultimate self-contained traveller.
Backpacking as a mainstream activity got going in the late 60's/early 70's. A magazine dedicated to backpacking came out in 1973. http://en.wikipedia.org/wiki/Backpacker_(magazine) It does not and never has included articles on schlepping around Europe between the train station and the hostel with a backpack.
So to answer the question of whether backpacking with a suitcase could be considered 'real backpacking', the answer is no but then neither is what most call backpacking, real backpacking.