Great question... we sell a lot of break cues through our retail store, and have noticed over the years how most break cues tend to have 13.25 - 14mm shafts. This lends the phenolic tip to have a more gradual radius. While phenolic tips don't really need as much scuffing as leather, they do need to be etched with texture from time to time, and the quarter radius fits the profile well.
Also, the quarter radius will allow those who like to "custom" shape their playing tip a more open approach. They can use the tool to tweak the radius instead of following a strict dime or nickel by scuffing at a chosen angle.
The scuffing radius area of the tool uses a proprietary grit of aluminum oxide canvas backed abrasive... if I tell you any more... well, you know the drill
-April