Bottom line: You can get good needles that perform as well (or better) than LM needles for much cheaper.
That being said, I love my LM needles and wouldn't trade them in for anything. While I use my KnitPicks Options needles for most things, I relish (and look for) opportunities to knit on straight needles with my Lantern Moon needles.
The reason is simple, sheer luxury. I love the feel and the look of wood. The LM needles are just beautiful and they add to my pleasure while knitting.
So you probably can't justify the purchase on any level except wanting to treat yourself. Metal and bamboo are much cheaper and perform just as well. But if you love the look/feel of quality wood and want to 'up' your knitting enjoyment, LM needles are fantastic.
And while you're at it, you might want to check out this link:
I haven't bought any of these needles yet but I'm definitely looking seriously at them.