MT, OK, since you ask. The National Training System (NTS/BEST) describes a good string hand hook as follows. I've really shortened what is a lengthy discussion. At pre-draw, the fingers pressures are 70-80% of the weight on the index finger. At full draw, the pressures change and are ABOUT 40% index finger, 50% middle finger and about 10% ring finger. The ring finger is mainly used to "...stabilize the hand on the string and resist unwanted twisting."
The reasoning is really that for shooting at 70 to 90 meters, EVERY little aid to totally consistent "everything" is necessary. So the ring finger is used to help assure a really consistent string hand angle.
As far as our kind of shooting goes, the ring finger may not be quite so important and as you can see above, most of the weight is on the index and middle fingers to start with. Can you shoot with just the two fingers? Absolutely! I'd suggest, though that any help at reducing stress where ever we can is a plus. Thus even that 10% weight on the ring finger will reduce stress on the other two fingers.
Do we as trad shooters really need this kind of exactness for our usual 20 yard shots? Probably not. I have seen, too, where many (not so exact) shooters actually introduce string twist with that ring finger. This is caused by way too much tension in the string forearm and wrist.
When I am coaching a student, this is something I REALLY try to get them to do -- use all three fingers.
I really suspect, that the shooters that really find that two fingers is working better for them is that that style is covering up other flaws that should be addressed such as too much tension on the drawing side. I'd describe this as a "work around" that may work but does not really address the problem.
Well, you asked.