I think it all depends on your smell tolerance, Catherine. Because while it’s certainly true that having a dry change of long underwear at night is a good thinghell, even a great thingit really isn’t essential for hiking. The only truly limiting factor will be the amount of odor buildup over the course of a few days. But then, when I was climbing Denali several years back, I wore the same long underwear for the better part of three weeks. So did my climbing partner, Tim. And neither of us fainted from noxious fumes (the long underwear was tossed and never used again, however).
Silkweight T-shirt
As for fleece jackets and the like, there’s absolutely no point in carrying a spare. The stuff dries quickly, and it’s overkill to have an extra when you’re backpacking.
Still, for summer use there is one thing I like to do, and it doesn’t add much weight to the kit: I pack in a Patagonia Silkweight T-shirt ($30; www.patagonia.com), plus bottoms made from the company’s same Capilene fabric ($32). I try to reserve this setup for nighttime, although of course it can come in handy during the day too. But having clean sleepwear does help keep the sleeping bag cleaner, plus it’s just a little more comfortable. And the light Patagonia stuff weighs just ounces and takes up as much space as a pair of socks.