"Do we need the colons? That is, if we leave them off, is there anything that doesn't work?"
Yes! It's necessary to distinguish between keyword args and normal args:
(foo x y :a 3 :b 4)
Sure, you could define keyword args to be separate... but I like the idea of unifying them, since they serve such a similar purpose. Plus, I like that there's a syntatic difference between a list that is used for sequential lookup, and a list that's used for key/value lookup. You may disagree.
In any case, you can freely mix alists, plists, plists with keyword symbols, and ordinary tables in your own code. The question of this post is: should keyword plists be supported at all?
I'm leaning toward yes, assuming we can solve the questions I presented. And then, rather than making data types tables, I could make them keyword plists, which is probably better since the number of keys is usually small.
