Heh! This is a funny thread!
A few years ago when I got my first DSLR, the 30D, I quickly began doing a lot of bird shooting, so using AI Servo became routine. I also heard about the BBF setting, and began using that. It is useful, especially for those who "work" the technique, and so when my 5DC came and then my 1DM3 came, they were set up with that. And, especially with my 1D3, it rarely went off of AI Servo. And, I didn't want to hassle with switching things back and forth, so BBF and AI Servo is where things remained and have served me well.
All that being said, I wanted to make a couple remarks, observations from some "messing around" I've done.
One is about AI Focus. It was mentioned that our AI Focus is "notoriously bad", but in actuality I'm not so sure about that. Yes, it has a "notoriously bad reputation", and in fact I've never tried to "seriously" use it, but I've messed with it a bit and have wondered about it.
AI Focus begins in One Shot mode. It requires a focus "lock" before it can begin to function. So, it would not be a good setting for a subject that is in motion. You want to use it if a subject is stationary but may go into movement. Once it locks focus, as long as you keep the AF active, you can shoot like you were in One Shot mode, but if the subject moves the camera responds by switching to AI Servo AF.
Now maybe the switchover lags a bit, I don't know, but in tests I've done it seems to actually do "as advertised". In fact, I suspect that one reason so many people have "written off" AI Focus is because they are misusing it by trying to shoot moving subjects with it, but that's just a guess.
Another "point": it was stated that the Red Dot focus confirmation doesn't work with BBF and that it doesn't show up in DPP.
Well, that's a mistake. What actually happens is that the Red Dot confirmation/focus lock works fine in One Shot AF and for the initial AF in AI Focus AF and, if you take that initial shot, you get the confirmation and lock and in DPP the focus point will in fact show up.
However, if you switch to AI Servo mode with either the shutter focus or the BBF, there will be no focus confirmation/lock, simply because the camera won't do that in AI Servo AF! It won't "lock" focus, it's continually "recalculating" focus, so in DPP there won't be a Focus Point you can View!
I can't check all this stuff on a newer body, so if you are unclear on what I've said, please check it out yourself on your body, and let us know if there is any variation out there. Try the three AF modes using the shutter button and see if what I say is true, switch to the back button and see if what I say is true!
And then, a third point!
As I said, I've left my cameras typically in AI Servo, BBF, continuous shooting for short bursts, etc, etc. I have on occasion switched to One Shot because if I'm shooting stationary subjects, well, I like the focus confirmation/lock, even though I have no problem using AI Servo with a stationary subject and just letting up on the back button to get a "lock".
But two things came up over the past year!
First, last summer my 5DC has its mirror fall off, and in it went for the replacement. I still had my 1D3, and even my 30D saw some use during the three weeks the 5D was gone. But then it came back.
Well, I quickly found out that Canon serice does indeed set cameras to factory defaults. I first noticed this because my Back Button Focus didn't work! Even worse was the day I rushed off to do some shooting at an event and didn't have the "stuff" packed, including a spare memory card, but as I began shooting the camera was shooting away and it wasn't until I stopped to do some chimping that I found out I had no card, and the camera was set to the default, to shoot without a card! Crazy!
Anyway, as far as the Shutter/Back Button AF, I just decided to leave it be for the time, just decided not to fuss with it, because for any wildlife/birding/action, well I use my 1D for that, always in AI Servo, Always BBF!
Well, fast forward to the past few months, and due to financial needs I decided to sell, well, the 1D3, the 24-70, the 70-200 f/2.8 IS, and also my 85 f/1.2, pretty much all my "heavy duty" gear.
I still have the 5D and the 30D and, for critters, the 100-400. So, I'll need to rethink whether to move to the back button or just to say "Hey, we're having fun!"
More a challenge, though, is to make sure I'm in AI Servo! a month ago I was at a meetup with some fellow POTN photogs in downtown Portland, OR. For most of my stuff I was perfectly happy using the shutter button, and One Shot AF, but then, I had to switch over to catch shots of the "Lightrail" transit train, and we went down to the Waterfront where there are not only a lot of people walking but also bicyclists and other contraptions, so AI Servo.
Anyway, the good news is that my 5DC can actually perform in AI Servo! But dang, you can't see the little Red Dots in DPP!!!