I know that the standard scheme has already been chosen, and I'm not posting this as an alternative to replace the current system. I've already argued for this system but I think most people don't use it, and it wasn't really understood properly, so I just wanted to post about it.
The issue of which letters are assigned to which groups isn't discussed here because it's not relevant to the system for grouping the stickers.
Here's an animated picture of how it works:
How it works: draw a line from the center to the corner. Imagine a small circle (radius less than the line drawn) centered on the corner. Following this corner clockwise leads to an edge. Form a triangle using this edge, the corner and the center (see the picture). All the stickers which are partially inside this triangle are grouped to use the same letter.
The property of this that I like is that commutators for different pieces are very similar.
For example, taking the corners UBR, FUR and LFD, the matching edges with the same letters are UB, FU, LF. The corner cycle is solved with [R', U L' U'], and the edge cycle with [M, U L' U]. Since the centers are basically extensions of the corners, the same applies to them. The difference between these two commutators is moving the interchange slice, R, inwards to become the M slice. However using the Speffz notation, the corner cycle would be matched with the edge cycle UR->FR->LD. The edge commutator here would be [R', E' L' E]. This cycle still is similar but it
feels different, although that is probably just personal bias.
I think this would be useful to someone who is using the same letter for their buffers (UFL and UF for example) because the cycles from those position are similar. The same property does apply to Speffz but in my opinion it is to a lesser degree.
On a related note, if you write corners using a clockwise notation (UBR instead of URB), then the first two letters of the corner make up the notation for the edge. Changing the last letter from the corner to lowercase also gives the wing. Changing the last two letters from the corner to lowercase gives the x-center, and changing the last letter from the edge to lowercase gives the +-center. This works for Speffz too but only if you adopt a counterclockwise notation.
I guess in the end it's still arbitrary anyway
