That's a good point ywen. Another way to look at things is to evaluate the needs of each photographic discipline and then look at the product offered by each respective manufacturer.
For landscape shooters there are generally two overwhelming considerations, resolution, so pixels, and dynamic range. Sony, Canon, and Nikon are all tops in this category by a fair margin as illustrated by the
DxO Mark sensor evaluations and to which those three are bested only by some of the medium format backs.
For sports/action, in addition to what ywen mentioned about professional support, speed and low light performance is the name of the game. All one has to do is have a look around the sidelines of any sporting event to best measure who are the leaders in this arena.
For commercial work resolution and colour accuracy are key. There's nothing in the DSLR market place at this point in time that meets that criteria better than the Nikon D3x, however many professionals will choose and be using medium format here for the absolute best in image quality.
Reportage runs the gamut with respects to what is required but things like reliabilty, durability, shutters rated to 300,000 cycles for instance, high ISO performance for low light shots, wide reaching support, and now video intigration, are all critical considerations when it's your job and your gear is your lifeline. Canon and Nikon don't have much competition when all these things combined are considered.
Now speaking from personal experiance, as someone who just three short years ago got bitten by the photographic bug, I can tell you that while gear doesn't define ones shots, choosing the right tools for what you want to do is important. I started out with a little Nikon D40. At that point I had no idea what I wanted to shoot, had no idea where photography would take me, but as I began to figure out what I liked I am happy I got into a system that offered a wide range of product options for me to grow into and choose something best matched my needs and wants.