Today I consider COMMA! to be an indispensible addition to the language.
But something that feels like a Rebol philosophy point is that we should strongly discourage the idea of commas having semantic meaning in dialects. The status quo should be that they just provide visual separation.
In this mindset, the only impact adding a comma should have on code would be an error if they're not put in an "interstitial" position. If you want a grouping to have semantic meaning, then a BLOCK! or GROUP! should be used... or perhaps a vertical bar |.
I think giving this guidance helps preserve Rebol's natural language stream aesthetic, where the kinds of problems mandatory commas introduce can be avoided. Whether you are doing things at source level or in code generation, having N entries needing (N - 1) commas can be an annoyance.