I can only think of a few instances where the forked combos really come into play, Namely monk wouldn't need for macro chains. Most chains have a start, middle and ender button, nothing more complex past that. Using attacks outside the combo is just something that never happens.

To give an example, right now Paladins have 5 keys for three combos, The Dot, The Damage and the enmity combo. You can reduce this to three keys with a key chain for each chain ender. Or two if you allow the enmity chain to change the last part to be either the D.O.T over Damage button upon using the middle button. This is mostly coming from a person that plays melee classes as well, where I find PvP chains to be oddly strange to just not be a choosable feature. If people want to stick to a five button thing, that's fine. But the option for others to tighten up there buttons and focus *oof on the macro play, and focus on the encounter, not if I accidentally used my ender instead of my middle would be a nice refresh.

Hence the push for a customizable macro for it then a full implementation across the board. Personally, I would just have it for select classes to just reduce the buttons overall. Some of them I won't use without another button previous (Like melee combos, and some actions for Archer -IE. The dots- )