Edit: @potench preev.com is an inaccurate indicator of the BTC/RUB rate because it is basing its calculation "based on the USD Bitcoin price" per its footnote, so of course it will never show a big deviation from the BTC/USD rate. For the true rate you need to look at the real-time bid/ask spread on a BTC/RUB exchange such as btc-e.com which is why I linked to https://bitcoinwisdom.com/markets/btce/btcrur or you can also look at https://btc-e.com/exchange/btc_rur: bid/ask is 20855/21099 RUB as of 22:09 UTC, so the arbitrage opportunity is approximately 2000 RUB.
If people think such a scenario can't easily happen again, then they're wrong about that. A small number of major bitcoin holders mostly determine the price.
You can't afford think that way. I did, and it took me a long time to realize that I was being insane while I was doing it. Unless you're planning for a timescale measured in "decades," you're setting yourself up to get burned.
Out of all the persuasive arguments I could make, consider this: There are better ways to double your money than bitcoin. Within the next decade, you probably won't see something like the leap from $6 to $600 again. I'll be happy to eat my words about that. Personally, I don't see how you'd even get to 10x your investment. $350 to $1200 would be 3.4x ROI. And while that's a good return, there are other ways to grow a nest egg from $20k to $68k without gambling it. And if we can't publicly admit that bitcoin is essentially gambling, not investing, then we should at least admit that to ourselves.
EDIT: One point I'd like to underscore is that the price of bitcoin is mostly determined by the major holders. If any of them lose faith and pull out, that will trigger others to pull out. Also, they have a lovely tendency to dump a lot of money into the market, which jumps the price, which causes other people to pour money in. Then the major player sells, capturing all of that money that everyone else poured in. You see? It's a predatory game, and you don't have to take my word for it. Examine the graphs and you'll see "jump-sells" like that over and over again.