Bushwacking and tough skin.

The natives had fire, a wonderful tool for clearing out underbrush, and used it regularly.

You can probably toughen your skin, or at least de-sensitize it; but I recall even Cody getting his legs cut up in some environments. Sorry, I don't have any good tips for it.

Just this last summer I walked around my local grasslands in shorts. There were some very coarse dry grasses and prickly plants, but I didn't mind at all
I have worn pants up until the past couple of weeks and with all of the warm weather we have been having I am now back in shorts. I have noticed that my legs are now very sensitive to even the slightest abrasion. Makes sense.

In my experience no. I've learned to cover up when bushwhacking as infection control in the wilderness is one of my top priorities for an enjoyable trip. One of my predecessors at work died of flesh eating bacteria after returning from the Philippines. Another man in NY lost his leg due to infection from what he thought was a very minor cut he received when he was harvesting firewood.

I don't think that trying to toughen your skin on arms and legs is a good idea. For one, I don't see how the skin in those areas can ever get as much use as your hands. Also, tough skin is great, but when you do cut or scrape it, it takes longer to heal. A cut on callused skin takes very long to heal- believe me…
I would stick to long sleeves. Besides protection from scrapes, you get protected from the sun and loose long sleeves keep you cooler than short sleeves in full sun anyway.

I know people with skin that tough, from intense and prolonged exposure to wind, sun, salt water, and extremes of heat and cold. Basically, they tan their own hides and turn them into leather (my father doesn't need oven mitts, it's kind of horrifying). Then they have trouble with drying and cracking, just like with leather. Scratch-resistant, yes, but they still get infections. And because it's true about slower healing, they are probably MORE likely to get infected once injured, and certainly more prone to scarring.

My general rule is to avoid cuts/infections on a trip if at all possible. If I know I'll be going through rough or thick brush it is always long pants and sleeves and if I do get a scratch or cut on exposed skin I disinfect right away. I love my little 3oz medical kit I carry in my pocket.

I'm probably a little more paranoid than most though, my father-in-law had a long cut from a rock underwater in a canoe trip got really bad by the time he came out a week later since he didn't stop to treat it properly.