I really think it depends on what level of mastery you're starting out at when you add the flash, and I'm probably hanging out on boards with a lot of amateur first-time dSLR shooters who got a flash their first week, but are on such low budgets, they went cheap 3rd-party and don't actually have real eTTL II functions.
Because if you don't actually understand exposure and metering, it's going to be hard to "fix" it when it doesn't come out quite right. FEC can only get you so far. And the default Av/Tv flash on the Canon side does tend to limit your ability to balance flash against ambient, unless you mess with the camera's custom functions. And even then, you're still going to have to balance either at the fill end or mostly-flash end of the spectrum.
Just as "P" mode makes it tougher for some people to learn what their camera's actually doing, because it's taking care of all the nasty implementation details, eTTL can sometimes obscure what's actually going on. And there's also the fact that if you are going to go forward to professional portraiture, then the same amount of money could get you to monolights instead--and at that point you're still going to have to learn manual lights. And speedlights are power-limited compared to monolights. Power with flash is like max. aperture on a lens.
Yes, but you
know how it all works. Are you honestly saying that you should ALWAYS shoot in Av with eTTL using nothing but FEC with off-camera flash at
all times? Because the
default settings on the Canon side are set for fill proportions and there are
a lot of very confused newbies who don't understand why, when they're shooting in low light, they still get a shutter speed long enough to get camera shake blur even though the flash went off.
Knowing WHY using a Custom function to shift your shutter speed with flash for Av mode to 1/200s makes the difference it does is kinda key on this one.