It's classified as a "sports" lens, so it would be silly to consider it for dedicated portraiture. With my monopod I'm sure I could use it for some type of portraiture at the wider end of the zoom range if I had to. But that's what smaller lenses are for. As for 300mm not being long enough for serious outdoor professional use, not sure I agree with that. It depends on how small the subject is, and how distant it is. Also depends on if you need the f/2.8 for low light. I would love that.
Again, the main advantage of this lens is its zoom capability and the f/2.8 capability through that range up to 300mm. No one else offers such a lens, it's unique.
When I am out wandering around doing shots of insects, wildflowers, birds, turkeys, deer, and whatever else is out there...it is nice to be able to zoom back and get wider shots. Sometimes you want to include the clouds in the sky above the wildlife, or the trees, water, or other landscape elements around the wildlife. With a zoom, you can both zoom in on the subject, and then get wider shots too...almost simultaneously. I certainly enjoy this feature with my 120-400.
In my opinion you could easily use the 120-300 f/2.8 lens for more distant wildlife work, provided it took the Sigma 1.4x TC well enough. Perhaps it's time for Sigma to offer a series 2 TC line. Best of all would be an excellent 2x or even 1.7x teleconverter.
It's really time for Sigma and Canon to embrace the idea of a 1.7x teleconverter, it seems to me.