There are not many practical or functional reasons to buy any of these lights (Arc6, HDS, Ra, Novatac, McLux, etc). They are connoisseurs' lights and the reason to buy them is satisfaction as an aficionado and/or to fulfill a desire. Your question is like asking for good reasons to buy 18 year old single malt Scotch. If you try to justify it in terms of nutritional value, you're fooling yourself. It is all about satisfying your discriminating tastes.
My reason for buying expensive flashlights goes something like this. We all put up with endless BS and compromises every day of our lives. But for a mere few hundred $$ of one's recreational funds, less than a typical computer upgrade or 1-weekend beach vacation would cost, you can get something that is the absolute best there is in its category, which you can carry with you and enjoy every day of the week without it getting you stuck in traffic. I realize $300 for a flashlight sounds like an awful lot to a non-flashaholic, but in the scheme of things (look at what computer gamers spend on overclocked CPU's and graphics cards), it's not that expensive a hobby, and the stuff does tend to hold its value. And, by buying from independent and semi-independent flashlight builders, I'm helping advance the state of the art of a really cool medium. So I feel ok about the expenditures.
One of my long term flashaholic desires is a Surefire M6, an obsolete and impractical light in many ways. But, I expect I will buy one anyway someday, just because I've always wanted one. If you find yourself desiring an Arc6, or an M6, or any other light, then as the saying goes, the only way to get rid of temptation is to yield to it. If you don't find yourself desiring it, then there are probably more satisfying things you can do with the same amount of cash, so asking other people to make a sales pitch for you is counterproductive.
you beat Marduke to pointing that out to me 
