That's if you don't trade at all. Otherwise the $600 are discounted at $50 per $1M of transactions which is rather easy to reach if you have a good system and enough capital with regards to trading frequency. But I agree it's definitely not a good idea to subscribe outside of these conditions.
$600 for every two months of testing with FXGame isn't as bad as $600 every month with a losing system.... but the Oanda folks haven't gotten back to me about this aspect. Do you think they would frown on someone cancelling and re-signing up for that purpose? We'll see how it goes. I've also been burned (in simulation) by systems that take advantage of high frequency anomalies in tick data from various sources. But, afterall, the more you trade, the more spreads you pay, so the higher the trading frequency - the more accurate your system needs to be overcome the costs... which is a rough road since the higher frequency components tend to more noisy and harder to predict. (EURUSD for sure).
I really don't know.. It may work a couple of times but after that they may get angry as you don't really play by their rules. But the higher the frequency, the faster you know if your system is profitable or not. And if you trade on a large number of currencies (say 15 tradable currencies on Oanda), the lesser your drawdown, as the result of (HF) trades among these will be more or less unrelated. Not speaking of larger potential profits. All in all certainly worth the effort.