One of the reasons professionals develop brand loyalty is (as mentioned above) the glass involved. Who said earlier the camera bodies grow obsolete quickly? Well, the glass doesn't. If you've been shooting for five years and need a new body, are you going to go get a new Cannon, or something that fits the 4 grand(or 10) worth of Nikon glass in your bag?
Another "brand factor" for the professional(or any long-term photog) is what you might call muscle memory. The menu/sub-menu/dial placement/etc tend to be similar across brand families. Changing camera brand involves re-learning all the stuff that has become second nature/automatic to you.
Personal story - traded from a Pentax ME-Super, to a Pentax P-30, to a Canon,.. all film - I'm old. Hardly noticed the difference between the two Pentax bodies. Shot the Canon for 4 years and never did manage to get the hang of it - I still hate that thing and it's left a bad taste in my mouth for all Canon products. Pretty sad, 'cause it's a good camera, just my inability to learn to use it.
I have a hunch cameras are like guns: The first good one you buy is going to mold your taste and preference, so be prepared to live with whatever brand camera you buy forever(not a bad thing overall)
Anyway, don't get too tied up in make of camera. Shoot what you've got. Don't buy cheap glass, as mentioned, it'll outlast the camera. Shoot the hell out of whatever you buy. One of the great things about digital pix - they don't cost a dime to develop.