I used to work for an Amazon merchant doing roughly $2 million /year in sales-- not big, but big enough that this was not a side business.
In our case, FBA (Fulfillment by Amazon) was useful for a few things, but increased profit was not one of them.
On the contrary, we used FBA to sell 'hot' items quickly, which in turn led to boosts of higher customer feedback for our merchant account.
Ultimately though, the increased traffic you might get with your FBA items is seriously offset by the increased competition you will get from other merchants and even Amazon itself. And After Amazon takes the cut for doing the fulfillment, you are left with pennies per sale in profit if you are lucky enough to get the volume sales.
Another unexpected negative aspect of using FBA is that occasionally Amazon fulfillment centers claim that there is "something wrong" with your goods or they are "damaged in shipment" and they refuse to store them or ship them. You have to eat the cost in these cases.
I think FBA can be useful if you are a merchant with a specific product line that won't experience direct competition, or if you are a small merchant trying to build some traction.
On a side note, I think FBA is a brilliant win-win strategy for Amazon: they get paid for storing stuff whether it sells or not, and they get merchants to test out product lines at ZERO cost to themselves.