Summary:
Why sleeping bag pads? Because waking up cold and sore is no fun. It's also no fun to carry a three pound sleeping pad, so here are some better options.
Sleeping bag pads are not one of the things you should cut from your list. Backpackers want to reduce their weight, but not even ultralight backpackers want to reduce their comfort. Still, who wants to carry those monstrous old inflatables down the trail? Try some of these lightweight options instead.
Make four-ounce sleeping bag pads, and sometimes carry two of them. Use the plain blue closed-cell foam pads available from any backpacking supplier. They are made larger than necessary, usually 24 by 72 inches. Just cut them down to a four-ounce size that fits your torso.
It should reach from your shoulders to your hips. You may want to cut the width a little at a time, testing for comfort as you go. The pad should be as small as you can make it, while still big enough to insulate your torso from the ground. A pillow of spare clothes can be used for your head, and your legs can be on your empty pack to insulate them.
Sleeping Bag Pads For Lightweight Fanatics
To make it really light, cut pieces out of the pad. Small holes in the pad don't seem to make it less comfortable. If you cut out a hundred little pieces of foam, you save an ounce and join the ranks of the fanatical ultralight backpackers.
Want to go even lighter? Leave the pad behind and try sleeping where the ground is soft. You can also pile up leaves or dry grass to sleep on. Do this where it won't harm the enviroment, and scatter the leaves in the morning so they don't kill the vegetation they're on. For fifteen minutes of work each night collecting materials, you can leave the sleeping pad home and actually be more comfortable. A thick pile of dried grass - now that's a nice camping mattress.
More Luxurious Sleeping Bag Pads
Want more cushioning? Inflatable sleeping pads are no longer out of the question for lightweight backpacking. The Big Agnes Air Core Pad from REI is a 3/4 length pad that weighs just 16 ounces and is an incredible 2 1/2" thick! If you've slept with Big Agnes, let me know how she is.
You can also find a few self-inflating pads that are reasonably light. The old Thermarest I sometimes use is actually only 21 ounces, but both Thermarest and others now have self-inflating sleeping pads that are under a pound. That's a lot of comfort for such lightweight sleeping bag pads.