Not that low-cuts don't have a place in the outdoors. They're light, agile, and comfortable, and some people can indeed get away with wearing them even with a pretty heavy pack. Better still, packs are getting lighter as gear makers find ways to shave off ounces and even pounds. So it's true that we need less ankle support than we did perhaps ten years ago. That said, I think most people under-boot a little, seeking comfort and forgetting that the boot also offers protection. My take on this, though, is probably in part due to my own infirmities. A high-school ankle injury suffered all those years ago came back to haunt me a while back and now requires frequent use of an ankle brace. So the thought of hiking in low-cuts makes me cringe.
Still, as you mention, not wearing a pack does make a difference. Even I still do a fair amount of boulder-hopping with trail runners or their equivalent. So you might take a look at a shoe such as the very popular Lowa Tempest Lo ($100; www.lowaboots.com), a low-cut shoe that has a surprising amount of heft. Very stable, too, which helps keep ankles upright. Montrail's TRS Comp ($95; www.montrail.com) is another stable, sturdy low-cut shoe that's popular with trail runners and light hikers. Another really good shoe for boulder-hopping is the Five Ten Mountain Master ($86; www.fiveten.com), which has super-grippy soles and a supportive polyurethane midsole.
That said, I also like many of the new-generation "mid-cut" boots, which are light yet offer a little more support. Among the best of these: Asolo's FSN 95 ($150; www.asolo.com), which has proven very popular, and includes a Gore-Tex liner as well. Tecnica's Spirit Mid GTX ($150; www.tecnicausa.com) sorta splits the difference between high- and low-cut boots. Very agile, but supportive.
Anyway, that's my take on it. Get yourself some trekking poles, toothey're wonderful for keeping your balance and protecting knees and ankles.