Canon EF-S 17-55mm VS Sigma 17-50mm VS Tamron 17-50mm?

You can continue to not know about it then, sorry, but it's true.  The ability to render a background completely out of focus is nice, can be done with zooms, and isn't done for all portraits, not even the majority of portraits.  Studio portraits are pretty much never taken this way, it's more useful for location shoots and events.

There are even pro portrait photographers out there using crop cameras, all things are possible, regardless of what you read here.  You can check out a lot of pro portrait techniques by signing up for photovision or the like.  While primes and thin DOF are used by some, they are not dominant, and not necessary.  Again, group shots don't play nicely with f1.2, and all lenses perform better stopped down a bit (though the 85L is impressive at f1.2).

Remember that DOF isn't entirely based upon aperture, focal length + aperture + relative distances matter.  Even using thin DOF you need to pay attention to the backdrop to achieve the best results.  Again, it's a tool to be used not to live by.