I've done that a few times (turning left before the oncoming traffic takes the intersection), but I'll most certainly agree that it is not usually a very courteous thing to do, and can be incredibly unsafe.
I've only ever done it at intersections that can be said to "favor" it - if they aren't perfect perpendicular intersections, one can can provide a long enough delay between right-of-way traffic "owning" the intersection that favors a bold left dash.
But anyone who does this regularly needs to be super certain they are paying enough attention. For myself, doing it relatively rarely, it's a combination of favorable factors. The right intersection shape, no bicycles or pedestrians to worry about anywhere (you can't predict those assholes so it doesn't matter where they are and what you THINK they'll do, if they are remotely in the area of concern, it's a no-go, always), and sleepy oncoming traffic.
At such favorable intersections, I always give a moment to the oncoming traffic - if they are sleepy at the line, I take it. At busy intersections with long lines at the left-turns, I feel this is beneficial to those behind me, and if done appropriately and with a tremendous amount of caution, then it's not a bad thing at all.
I don't think I've ever done it at a perfect perpendicular intersection like in the OP - that's just rude unless it's a single lane and the other driver is distracted.
In an unrelated situation, I once missed the opportunity to turn left because the driver in front of me (can't remember if he/she turned left or drove forward, it was one of those shared lanes at a small intersection), decided to completely not pay one bit of attention to the intersection. The light turned green... then yellow and red, and the driver only started to creep forward when it was just turning red.
I was livid. My last intersection before I get home, so it barely delayed me in the end, but that is beyond the point. It's a matter of principal, and was just one more driver on the lengthy list that piss me off on a daily basis around these parts. People are just so ignorant and clueless as to how traffic should flow and how they can play their part in maintaining a smooth traffic pattern even when it is busy. I like to speed, sure, but I know when it is more appropriate to yield in various situations so as to ensure traffic continues to flow, and not have to brake to merge into a certain lane to exit from one highway or anything like that. One or two instances like that is what causes mile-long backups. I see it all the time, the same exact traffic density, sometimes it flows beautifully, sometimes it's a gridlock. Almost always it is because of someone's boneheaded decisions, trying to anticipate opportunities or doing the exact opposite, being so incredibly passive and scared their reactions or overreactions screw everyone else. Hell, half of the accidents out there on the highway could be said to be caused directly/indirectly by drivers who escaped without any damage done to their vehicle or license status.
Ugh, these topics always bring out my ranting diatribes. I just so enjoy driving, which can be strange when more often than not it seems like the whole world is out to put me into the worst mood possible.
But half of my commutes have actually been smooth as of late... even with a ton of construction literally everywhere, I think I might just have the right commute hours or something. But I still encounter the assholes and dare I say retards who ought to have their licenses revoked. I am still in favor of mandatory re-certification. If tech certs are no longer a lifetime cert, why can't we require a renewal process for licenses? It would be awfully inconvenient, I agree, but it feels like something that outshines any other example that is said to be a "necessary evil."