Lightweight bars don't save ounces, but pounds...I've weighed them both. The saw balances a lot better and is quite a bit easier to use if you gotta do some brushing.
If I'm out falling timber, I carry just 1 saw. With a 28" lightweight, I have a manueverable enough bar to swamp out the base of a tree, yet it's long enough for most trees.
As far as what I see from production fallers, it seems like most of the local guys have or are in the process of switching over to lightweight bars.
No real difference in longevity either. They use the same sprocket nose as the standard bars. The bar rails are still steel, so the rails aren't going to wear faster. If anything, I could see lightweight bars helping put with longevity on the saw...a lost of west coast guys are running 32 and 36" bar's, that a lot of weight being supported at the studs. I figure less weight on them is a good thing.