While Bibble can't do layer operations, dodge/burn, and cloning like GIMP can, it can rotate, crop, sharpen, adjust levels, saturation, and EV. If you depend more on your camera for composition than your editor, Bibble will get you quite far in many cases.
The downfall of Bibble on Linux is that there isn't any good way to profile your monitor. Thinkgs like OptiCal (calibration puck) just aren't supported on Linux. So, even though Bibble supports ICC profile transformations, you can't get an accurate profile for your monitor under a Linux video driver. That pretty much ends the game before it begins.
I think usability of the GIMP is an excuse when it's in the context of interfaces, but anyone who thinks adjust levels in an 8bit color space is adequate has never seen an image adjusted in a 16 bit space. No more JPEG jaggies when I use Bibble.
GIMP is a fantastic tool, and I'd love nothing more than to use it exclusively. But until Xorg supports integrated color management, there's no point in GIMP having 16 precision. It's like trying to to precision machining with a tape measure.