I was a CNC machinist (mills, lathes, laser cutter, waterjet, press break, and turret punch) for 10 years and am a current Makerbot owner. For a 1 or 2 off item it takes far less time to print it on a Makerbot than it does to mill it on a CNC. I can design a fairly complicated part and print it on the Makerbot in less than half the time it takes to program the G-Code, set up the mill with the different tooling necessary, set the heights and radial offsets for each milling/drilling tool, and set the home position on a CNC mill. If you have to flip the part and machine multiple sides, rinse and repeat for each operation.
Makerbot parts are in fact very strong. I have designed radio controlled airplane engine mounts (gas engines) that are plenty strong enough to tow a plane around at 100 MPH. I also made a GoPro camera mount for my scuba diving mask.
To say that 3D printers are slow and can't make anything worth while is simply and completely false. Check out thingiverse.com sometime. While there are a lot of items on there that don't serve any purpose, look deeper and you'll find a plethora of practical and very useful items.