It's an age old question FF v's crop.
FF produces such nice images and depth of field in the images. The sensors are also typically higher resolution (albeit perhaps not per square mm of sensor) so you can crop hard into the images. The reality is that a lot of people are scared of cropping hard, and if they stopped worrying then they wouldn't really suffer too much for having FF.
People would be surprised how hard 'professional' images often are. If you can print it OK then whats the problem. The modern camera's like the 6d have such brilliant AF that they will handle a 1.4x TC quite well. So that is another option if you lack focal length on a compatible lens.
The 6d is a cracking little camera, I would have one in a heart beat if only it had more AF points near the edges. I must say the AF impressed me a lot for a consumer camera. Handled some conditions i don't think my 7d or 1d3 would have enjoyed.
The 6d would be plenty robust enough to withstand even light professional usage.