This is one of the most fascinating topics in cryptocurrency, in my opinion. There are so many aspects to it, and so much bias, that it always makes for an interesting discussion.
In my opinion, crypto-currency is poorly named. Crypto-coins definitely exist, as a quantity recorded on the associated block chain, but only very rarely do we use them as a direct measure of value, which is what a currency does.
This is where the English language sort of fails us, and sadly, I don't have another one to use, so I'll do my best to explain my take on the matter. In common parlance, we don't make a distinction between a paper note in your hand and the measure of value that it represents. We simply use the word "currency" for both. This isn't quite accurate, though, because there is a difference. A currency - a dollar, or a yen, or a euro, or a peso, is an abstract unit of measurement, much the same as a meter, or a second, or a liter. A meter stick isn't a meter, it's a stick one meter in length. I could give you a ribbon measuring one meter in length, but I could never give you an actual meter. Similarly, a dollar bill isn't a dollar, it's a bill, one dollar in value. I could never give you an actual dollar, only an item measuring one dollar in value.
The financial world uses the term "commodity" to distinguish between an item and it's value. For example, one bushel of corn (a commodity) currently has a value of $3.65. A cow (also a commodity) in my region has a value of $150. If I wanted to trade a cow for an equivalent value in corn, I'd have to ask for 41.09 bushels. I might make that trade, swapping one commodity for another using a currency (in this case, the dollar) as a measure of value for each item, without ever involving the use of dollar bills. Dollar bills are simply another commodity, useful because I can only fit so many cows and bushels of corn in my pocket.
Understanding this, it's fairly simple to see that cryptocoins don't generally act as a currency, but as a commodity. When we buy items using Bitcoin, the prices we pay (the measures of value) are almost always in a fiat currency. I don't pay 0.0216 BTC for 1 GHX valued at 0.0216 BTC. Rather, I pay $5 in Bitcoin for 1 GHX valued at $5. The currency of the transaction is in dollars, Bitcoin is simply a convenient commodity for making the exchange.
This, I think, is where the financial giants of the world see immediate utility in block chain technology. Exchanging value around the world is an enormous pain currently. A multitude of systems, processes, and middle men (like the US Federal Reserve) can cause a simple wire transfer to take days to complete, cost a significant amount in fees, and imposes risks associated with relative exchange rates and charge-backs. However, as we know, sending a properly designed cryptocoin anywhere in the world takes minutes, costs a pittance, and cannot be revoked. What bank, in it's right mind, wouldn't be interested in this technology?
Will banks attack Bitcoin? Perhaps one day, but I think it extremely unlikely that they'll mount a direct attack on the network. There is simply no reason for it. A bank doesn't want your loyalty or your obedience - it wants your money, and Bitcoin currently provides no significant threat to fulfilling that desire. It doesn't matter how much Bitcoin you buy and hold, because sooner or later you're going to use it for something, and once you do, all roads lead back to the bank. The banks have no more reason to attack the Bitcoin network than they have to attack gold or corn or cows.