I can't say for sure why AIP have decided to go with the wheel style hammer bearing. I'm not part of their design team so can't speak for them.
But from an engineering perspective, it makes total sense to replicate the bearing found on the G17. I think that bearing is much more reliable and much more fluent than the bearing found on the G18c.
For starters, the bearing will roll along the BBU as opposed to gliding along. THink of it like a wheel VS a sled on pavement. Without the proper lubrication (like say, snow) the sled will not run smoothly. Whereas a wheel will work regardless. This same principle applies to the different style hammer bearings found in the two Marui Glock platforms.
The trouble with running with the wheel bearing is already pointed out by Turok_T. It has a harder time humping over the bump on the BBU (like a speed bump). This is where all the friction comes from. Unless you modify the BBU a bit to create that smooth cycle.
With the rotary G18c style, the bearing rotates around that hump, which will have less friction. But as it moves further into the slide's cycle, it will wear faster because of the contact friction (like the sled in the pavement).
THe second reason why it will be more reliable to implement the wheel bearing is to eliminate an impact point on the hammer itself. A lot of times, the bearing on the G18c will actually break with a hard kicking pistol. When you add the full auto function to the mix, it will only be a matter of time before the bearing breaks. This is often the case during the winter time with the pot metal bearing becomes frigid in the colder weather and becomes easier to break...
Here's what I'm talking about
:
You can see that a corner has been sheared off and notice how out of position it is as it sits on the hammer. The bearing, at this point, just rotates. There's no longer a place for the nub on the hammer to stop the bearing from completely rotating. This will jam up the slide cycle because the other corners on the bearing will catch the BBU the wrong way. THe second thing is that when it breaks, there is nothing to keep it from springing out and dropping to the ground....
This why I sometimes order the KM Hammer set. Just to get that piece in steel so it won't self destruct. The stock one is made of pot metal.
The trouble with the AIP set is that the bearing is made too big, causing a lot of friction. THis is why Turok_T has to come up a guide to fit it properly. For me, I replaced the bearing to a smaller, custom made one. The set isn't really compatible with any of the 8mm and 9mm bearings made by...Action(?) (I think).
So just be prepared to do a bit of work when you get yours.