Ask HN: Why not link AND text? xor vs. or [meta]

It is becoming quite a common thing (specially with the Show HN) that when you submit a link you'd also submit a comment explaining more about what you made, relevance to HN, specific questions, etc. So my question is simple:

When you submit a story you can choose "url" or "text", but why not both? What are the disadvantages of just setting them as optional? (and requiring at least one)

Meta: I'd suggest changing the "or" to "xor" to display this mutual exclusivity. Or just leave it as "or" and allow both