Well, from a fashion sense, my internal-frame packs look to me like what they are: climbing gear. My external-frame pack looks to me like what it is: a high sierra backpack. I've used both for both activities, and the trusty Kelty (pictured to the left here) is still in service.
I am keenly aware that my Tioga is heavier than virtually all of the awesome internal/no-frame packs popular with UL backpackers and on this site. I am also keenly aware that, although it CAN easily carry 60lb of gear or deer meat, I don't like doing that anymore. That load capacity is not why I prefer the Kelty and not why I suggest the adjustable-frame Kelty externals to my Scouts.
I prefer the external frame packs because of the way they keep the load directly off the the spine, the ventilation behind the back, the stability of the packbag on trails (not climbing ladders or rocks or scree... not saying that), the ability to manage weight onto the hips, simplicity of strapping on gear and capacity/design to accommodate things like bear canisters.
I used to marvel at the cool and light Coleman Peak 1, flexible plastic external frame packs, particularly at their silence...a Kelty doesn't squeak like a Jansport but will inevitably require one to learn the skills with appropriate use of a crayon or candle. I've noted that those Coleman frames many of us recognize from the early 80's appear to be on other-branded packs I see on Sierra Trading every once in a while -- I need to get a weight on those. They aren't adjustable, but I recall them as being light.
One thing for certain, the not-trendy, not-UL external frame packs are not very BPL-core. Even REI doesn't carry them in our local store (they keep an old external with hideous straps as an exemplar to sell other packs, killin' me). It is true that these obsolete boat anchors weigh more, empty, than some hikers bags with their sleep systems in it. I have fantasized, briefly, about asking someone like Dan McHale to replicate my Tioga bag in some super-cool Dyneema grid or whatnot, but just can't cut the cord with my old buddy. For now, I remain happy, upright and proud of the old rig.