Hindsight is always twenty-twenty, which can be a byatch sometimes. If you're actively daytrading (hopefully on an exchange that doesn't lag badly when trade volume surpasses 2000 satoshi per hour ), selling hoping to buy back profitably at a (significantly) lower price is a perfectly reasonable strategy. I'm not having a go at those who know what they're doing and attempt this (I was doing it myself on BTC-e not so many moons ago); the only beef I have is if anyone is naïve enough to believe that such strategies aren't gambling (because you don't know the price will dip x per cent until after the fact). Same deal applies to buying at a loss on the way up from a trough in which one sold low.
In the bitcoin marketplace, history tends to suggest that a simple buy and hold strategy (cold storage, we love you), while perhaps far from perfectly optimal, is probably still by far the best (and least stressful) route for non-super-expert daytraders. As for the exchange rate determining buying power when paying BTC to a BitPay site, etc., yes, obviously the $ price is important there, but TBH I was spending my bitcoin when I wanted/needed to, even when the exchange rate was <$15. I actually chuckle about recently buying a whole year's VPN abonnement for about 320 mBTC, when I paid 530 mBTC for a month's access with the same company in January!
But, if you buy and hold BTC1, then the price gyrates on Gox, you still always have BTC1. Another analogy: you buy a gold nugget for $600. You are now $600 poorer and one gold nugget richer. So if the official (scoff) spot price of that gold nugget changes to $700 or $500 whatever... nothing has changed, see preceding bold statement, which still applies.
Welcome to Bitcoinia, buckle up and enjoy the ride.