I have a heavy aluminum full size tripod, Giottos MT9360, with long ground spikes. Once i get that planted, i've never had to use weight to get sharp images. On the other hand, I have a carbon fiber small Manfrotto 190csx tripod, or some such. I've gotten soft pictures out of the Manfrotto tripod if i didn't have a weight on the tripod. (the Monfrotto doesn't have spikes with it)
So it all depends on the weight of the tripod, whether it has spikes or not, whether the ground is soft and spongy, like a forest floor, etc. as to whether you need weight or not. Tying oneself down to step on a bungy cord doesn't sound useful to me, particularly if its a long exposure like 20 minutes or more. But bringing along a cord that you can tie to a stone, tree limb, roots on a tree, etc. is all feasible. You can also bring along one of these canvas or fabric reusable grocery bags and load it down with smaller rocks, if thats all you can find. But one needs a hook or tiedown point on the tripod.
My $.02