I have a question to your all: What are the circumstances when you have to leave your bike unattended for, say, more than 3 hours, to warrant expensive and heavy locks? (Beside the possibility that you have nowhere to keep your bike at home, so it has to be outside all the time.) Examples: shopping, going to movies...
For me, because I hate carrying heavy chains/kryptonites, I try not to leave my bike unattended as much as I can. If I leave it out of sight in an insecure area it will usually be only a few minutes. I'm Ok locking it with light locks if the area/time is not very prone to bike theft with tools, or if I can always check my bike from a window. If I just go for a ride and don't plan to park the bike during my ride, I simply bring a cable lock for a quick run to a drugstore or toilet on my way.
Bottom line, it seems to me those heavy locks are necessary only if (1) you leave your bike unattended in an theft-prone area for a relatively long time, (2) your bike is expensive/new.
Of course, this opinion comes from someone who what to lighten up as much as possible, and who has yet to be a victim of bike theft.
Well if you live in a high crime area(NYC,chicago,etc)where there are professional bike thieves,leaving your bike even for just a sec with a cheap cable lock and you are asking to have it stolen.Now I live on the "outskirts"of the hood,but when I commute I go through some sketchy areas and when I stop somewhere,even for a second I want to know it will be there when I return.So I guess ,for me anyway,I am using these and carrying these for peace of mind.Now I do have a crappy cable lock that I will use occasionaly when Im riding and not expecting to be stopping any where,but when I ride I mostly dont have too much of an agenda.I could end up at the coffee shop,the grocery store,or locking my bike up while I hop off to get a few miles in running.Either way,I wanna make sure my bike is there,plus all the extra weight just makes me stronger.