MicrostockGroup

No it's not a new feature. As long as you have activated several currencies to your account, a money transfer stays in its currency's balance until you cash-out or you ask for a conversion.
If you receive some money in a currency for which you don't have a balance, it's automatically converted to your default currency.
You can convert it at any time, so yes, you can wait for the best rate. IIRC, you can convert from any currency balance to any other one, but you can only cash out by converting to your main currency.

[EDIT] And another advantage is that you can keep some money in a foreign currency when you need to buy something abroad, without paying twice for currency exchange.