I wonder why Canon uses the pressing an assigned button continually to toggle back and forth between the two method instead of just pressing and releasing to get the desired drive mode. An icon in the viewfinder to show which drive you are in would be a nice touch but I could live without that.
I can access this in the Q screen but I'm often on the FEC section and have to move around to get to the drive section and then back to the FEC. In a fast paced environment it is pain to hold the DOF or any other assigned button continually when you need the opposite drive. With modern electronics and programming I'm not sure why this can't be done. Even giving the option to select continuous holding or one press could not be that tough.
How about a FW update for that?.
I think it might be a good idea to mention what camera(s) you are talking about & what your concern is. I rarely take my cameras out of AI Servo even if shooting stationary objects. This has never caused any misfocused shots to date or if it did I didn't notice it.
I have the 7D and 5D3. I've owned all the XXD lines from the 20D up and both previous 5D's. I'm not having any problems. When I'm in a dark venue I prefer to see the focus lock illuminate. If I'm shooting the bride walking down the ilseway I use AI Servo but prefer one shot when shooting static. I find after a long day I tire out trying to determine if AI Servo has locked on what I want it to lock on. Whit one shot I toggle to the AF point I want and get a confrimation. Just a personal preference.
LIke the previous poster, I just use AI Servo most of the time... only change to One Shot on rare occasion when I want to insure really accurate focus with a stationary subject.
However, I believe it might be possible to assign a button on some models to more quickly switch between the two modes.... Not entirely sure whether 7D or 5DIII can be set up that way, you'll have to research it. I don't have any problem changing using the standard procedure.