Great question... You know as I was writing the above, I can't help think that the half-butterfly is really quite influential in the development of the modern pro-fly style. What I mean was that the evolution was kinda like this:
Standup --> classic butterfly --> hybrid --> profly
As opposed to:
Standup --> Hybrid and separately Classic Butterfly --> profly
Why do I say that? If you think about the position of the non-saving leg in a half-butterfly, it is exactly in the same position as it would be in a profly. The knee is straight down... the ankle is allowed to pronate naturally so the heel points to the sky and toes to the ice. The only difference is that the saving leg is outstretched with your knee off the ice.
Now let's say you were down in half butterfly using old gear. To get up, you would dig in the skate edge of your outstretched saving leg and stand up. If another shot quickly came to the opposite side, you'd drop again, this time on the opposite knee and kick out the opposite leg. "Do that five times fast" as they say, and before you know it you are skipping the getting up part and just "butterfly" sliding across the net to make that save in the same way a modern profly-er would.
As for your question... many guys who use the profly also use the half-butterfly. It really just comes down to flexibility. Think about it this way. Two profly-ers are down in a perfectly square butterfly. Knees have even weight on the ice, pads rotated and fully sealed. A shot, perhaps deflected, comes to the left side... The "pure" (more flexible) proflyer will flash his/her left leg out, without breaking the seal of his pad on the ice.
How? Essentially he/she just rotates his leg out at the hip, using his knee as the fulcrum and swats the puck away. While it looks like he just made a "toe" save, he *actually* made the save with the outside of his ankle. (hard to visualize I know, but imagine being a kid, walking beside your brother/sister and smacking their butt with the outside of your ankle... same move).
The other guy... well he just lifts his saving knee off the ice and litterally kicks at the puck with his toe... his pad will instantly rotate back to "square" on his leg and the effect is the same as the pure pro-fly guy, except that there will now be space under his leg on the saving side. The advantage this guy has is that he can reach his leg further than the "pure" guy since he's now stretching from the groin as opposed to reaching the limit of his hip rotation/flexibility. The disadvantage is that he needs to be a lot more careful with his stick positioning to ensure the puck doesn't squeeze under him... but he can also control his rebounds a bit better.
We'd probably say goalie #2 is more "hybrid" in his profly style. I'm less flexible and this is what I tend to do. Really... the only difference (and I had to train myself to do this as an adult when I got my first 'profly' style pads)... is that the old-school half butterfly save, you dropped right into position. The new "profly" adaptation of the same move is to drop into your butterfly and then kick your leg out to "half butterfly" if and only if you need to reach the puck.
With good positioning then, and an absence of deflections, there is no need for the half-butterfly but it certainly is an advantage to have it in your repertoire.
To make things even easier on the hips, you can use a "sliding" toe bridge, replace laces with bungees, or get a "toe-hook" to attach pads to skates. All of which allow you more freedom to rotate your ankle.
j/k
