If you're looking to invest in currency ETFs, you better know what you're doing. They can be wildly volatile, but I would say that perhaps a small holding would be okay, especially if you're looking to diversify away from the U.S. dollar. Currencies go up and down, get stronger and weaker and I'm not sure the dollar will be around for too much longer. It may not be a bad idea to get away from it a little. Especially since so many equities and bonds are U.S. dollar denominated. The question is, which currency ETF do you buy? I don't have the answer for that. I'm not sure if there's a "basket" of currencies to use as a hedge. You'll need to find that out.
I used to trade currency ETFs a little about a decade ago. I can remember buying some Canadian Dollar (FXC), Australian Dollar (FXA), Swiss Franc (FXF), and Chinese Yuan (CYB). Some of these actually gave me a scheduled return. I don't think it was a dividend, so it must have been some sort of capital appreciation. I didn't lose any money on these funds, but after a while I felt as though I was being a speculator as opposed to an investor. And foreign exchange ETFs certainly part of my portfolio that I'd rebalance every so often.
Overall, there are some positive aspects of the currency market. It's very large and liquid, it can help you diversify away from the U.S. dollar, you can trade shares 24 hours a day (I'm not sure about that for the ETFs), and it's relatively inexpensive (again, I think ETFs have the same trading commission as any other ETF). There are also some risks involved with currency trading. It's a high leverage sport and it can be very volatile. So if you do anything, I would definitely stick with the ETFs as you suggested and stay away from diving straight into the forex market.