There is really no disadvantage except that you have another card. I do have both cards. If you manage your money well I would go ahead and get it. Also tie your reward points into it. Reward points from both cards will accumulate together. It gives you the option of paying over a couple of months in the event you have to. The interest rate on green, gold or platinum Amex is too high for purchases that qualify for long term payments.
If you don't have an Amex green reward card or if you currently use a Visa card or have a number of other credit cards I would not get it. There is no sense in getting a credit card just to have it.