If you're commuting and have to lock up outside, you can always leave a large ulock/heavy chain on some bike rack nearby where you lock up, so even if the rack you use is full, you still have a large lock to go around something else, without having to carry around some 10+ pound lock on you. Not to mention you have a super heavy duty lock for someplace you're going to be leaving outside for several hours daily. However, if you're not going to be keeping it locked up for hours in a risky area, there's usually some kind of rack/meter/metal railing/bum/fence (the posts, not the fencing part! and even then, they could still cut the fencing apart and lift your bike off the top if they really wanted to) that I can lock to within a block of wherever I'm going, at least around where I live.
I suppose out in more suburban areas there's less stuff to lock to, but at least in urban areas there should be all kinds of random immobile crap you can lock to.