I'm gonna put another example, just in case somebody still doesn't get it.
Dumb approach (4 buttons per ability to arena123):
Macro 1:
/cast [@target, nomod] Polymorph
/cast [@focus, mod: lshift] Polymorph
Macro 2:
/cast [@arena1] Polymorph
Macro 3:
/cast [@arena2] Polymorph
Macro 4:
/cast [@arena3] Polymorph
Intelligent approach (1 button per ability to arena123):
Macro 1 (the only one):
/cast [@target, nomod] Polymorph
/cast [@focus, mod: lshift] Polymorph
/cast [@arena1, mod: rshift] Polymorph (rshift remapped to NAGA1 for example)
/cast [@arena2, mod: rctrl] Polymorph (rctrl remapped to NAGA2 for example)
/cast [@arena3, mod: ralt] Polymorph (ralt remapped to NAGA3 for example)
The button/macro bloat only exists if you use the "Dumb approach", people need to learn that you can remap keys to modifiers and that there are more modifiers than they think.