Regarding butane leakage:
I have not had this problem with butane lighters, even after many years in storage. Case in point: when child-proof lighters became mandatory, I bought a whole box of loose lighters on clearance the day before they had to be thrown out. It's been 7-8 years, and I'm still pulling lighters from this stash. They're always full.

I mostly use Bics, which have proven themselves highly reliable in my experience, though sometimes I use Crickets for the adjustable flame. I wouldn't trust some of the "generic" brands for life-saving use -- this is not the place to go cheap.

In use, however, I have noticed that the mini Bics I constantly carry in my pants pocket do run down to 1/3 to 1/4 after a year or so. Part of that is from occasional use, of course, but I strongly suspect the valve gets tapped by other "pocket junk" on occasion. The warmth factor may also be a big contributor by increasing the internal pressure, meaning that you lose more fuel when the valve is tapped, and any micro-leak out the valve is made worse.

Regarding freezing temperatures:
I have been relying on Bics and Crickets for subzero work for 20 years. Keep them warm with body heat and they'll light at -25 C. I carry a Mini in an inside pants pocket at all times, and usually have a full-size spare in every jacket, pack, and glove box. They're so cheap and lightweight that there's no excuse not to have multiple redundancies. A minute or two of warming inside a mitt (or against your skin inside your belt -- yikes, that's cold) will bring a frozen Bic back to life.

Cheers,
Another Doug