Just to add to the point, there are many levels of what can be called 'professional'...there are career photographers with contracts who earn regular salary/pay, there are freelancers who can be very successful earning a living, there are freelancers not quite earning a healthy living but devoted to the cause, and there are those who don't earn nearly enough to live on, and use photography as a supplementary income. How much camera or lens any individual needs isn't just based on the label 'professional', but on their very specific types of shooting, and how often that type of shooting must be done.
No question, those in the first category should get themselves into the Canon or Nikon system, and never look back - those two companies have full pro bodies built for ridiculous abuse and high shutter actuations, with full pro support with dedicated lines, overnight lens replacement on-site, and more. I think we can all agree this class of photographer likely represents all of roughly 1% of the photograph-taking world. Or less. It probably represents .0001% of all Disboard members, or less.
Those in the second category would probably still need to stick with Canon or Nikon - their only source of income still comes from photography, and they cannot risk any downtime - the pro support would be the biggest selling factor, along with specialty lens needs that would be hard to find or service with any other brand. I'd venture a guess that we're still under 5% of the total photograph-taking world, and likely still less than 1% of Disboards for this class of photographer.
Once you get into the other 'professional' class of photographer, the need to stick with Canon or Nikon pretty much fades to a choice or desire, rather than a necessity. They have advantages, unquestionably - they have more bodies to choose from, more lenses currently available to choose from, and easier access to rental lenses or sharing/borrowing lenses from others, since more people are shooting with those two brands. They make fine cameras that are very capable. However, the other brands have their advantages too, and a professional who isn't in the top tiers might find the advantages of Pentax or Sony suit their needs well, even better than Canon or Nikon. They too make fine cameras, and have specialized features like in-body stabilization or fast live view systems with phase detect AF, that can make them stand out against competitors. And though not as many current lenses, both Pentax and Sony have good legacy lenses available with good backwards compatibility, including that in-body stabilization perk which allows even old manual lenses or fast primes to still get stabilized.
I am a 'professional' photographer, by other than the top 2 definitions above. I earn money from photography, but don't earn a living with it. I have found for my needs that the Sony DSLR system works perfectly - the advantages of the system outweigh the disadvantages for me, and I have been able to satisfy my need for very good 'pro' level glass by dipping into the Minolta G APO lens legacy, which was Minolta's pro level glass similar to Canon's L glass - heavy metal bodies, top-grade optics, lots of adjustability. I can keep my large lens collection, and move to different bodies (as I've already done once) to move up to the next evolution of technology, in order to get things like high ISO capabilities and burst speed advances (my first body couldn't shoot well above ISO1600 and was 3fps...my current body shoots well to ISO6400 and can go up to 7fps).
Just to put things in perspective. I often see reference made to moving away from the 'entry-level' as being a sign of a beginner becoming a real photographer, and how once a person gets to this level of skill and realization, they're only going to be well served by a Canon or a Nikon. Often, this advice generalizes by classifying all photographers in only two categories - beginner, or career professional. It misses the many levels in between, of advanced beginners, growing enthusiasts, super-skilled non-professionals, light pros, secondary-income pros, freelance pros for social events, freelance periodical staff photographers, and right on up to full pro sports photogs or nature/wildlife photogs. The line that cuts off all other brands, and leaves Canon and Nikon firmly as the only two reasonable or rational choices, is much higher up than some may be led to believe - one can grow to far beyond a beginner, and even become a 'professional photographer', with Olympus, Sony, and Pentax as well, and even do so successfully, happily, and in some respects, even better than they might do with the big two brands...with room to step up bodies or start moving into serious lenses. Only once someone moves to the pro leagues where your living is fully dependent on your photography, where professional support is vital to keeping your job or putting food on the table, or where your photographic needs become so specialized that only one brand even makes the specific lens or camera function that you need, does one need to start crossing other brands off their list, and stick with the big two.
That was for the Chocolate or Beige.
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