I am going to outline some reasons that a person might want to consider using an external frame pack rather than the ever-popular internal frames or frameless packs. Keep in mind that the gear that you pick should be gear that suits your individual needs and purposes rather than simply being something someone recommended or because everyone else is using it. What works well for some, or even for many, might not suit your particular needs.
As a person with Parkinson's disease, I have a variety of issues that I have to deal with that affects my gear choices. With regard to pack choices, they include lower back pain due to increased muscle tension; and excessive sweating (which is probably related to the medicine that I have to take). The problem with the excessive sweating is that I have to drink more water, which means I have to carry more water.
Beyond the issues caused by my disease, I am also a big guy - standing 6'2", 205 lbs, with broad shoulders and a 22" torso. As such, I need bigger clothes, bigger sleeping pads, bigger sleeping bags or quilts, etc. Consequently, the things that I pack tend to weigh more than an average person's gear and take up more space. Which means that I need a slightly bigger pack with a higher weight carrying capacity to carry the extra weight plus extra water so that I don't dehydrate. And my pack needs to carry the weight well, transferring most of the weight to my hips and lower body rather than my back.
I tried an internal frame pack, but found that it wasn't very comfortable and was extremely hot on my back. Eventually I ended up with a Kelty Trekker 65 external frame and am pretty satisfied. After stripping it down (removed the top bar, the inside "stay-open" bar, etc.), the empty pack weight is now down to 4lb. 9oz. - which is comparable to some of the bigger internal frame packs. It has plenty of room for my larger gear and effectively transfers almost all of the weight to my hips, which really saves my lower back. The mesh on the frame allows the pack to stay about 1" off of my back allowing air to circulate and keeping me cooler.
While it isn't the "latest and greatest" thing in backpacks, my external frame serves me well and suits my needs. So, if you're looking for a pack that can comfortably carry a wide range of loads, is flexible, keeps your back as cool as possible, and effectively transfers weight to your hips - you might want to give the external frame packs a try.
Shaker