Can you clarify how this better helps express swing compared to a piano roll?

You're not sequencing notes you're sequencing multiple parallel levels of musical information which are eventually rendered as notes.

One of those levels could be context-related microtiming - the context being position in the bar, or the actual shape of the phrase, the position in the phrase, and so on.

This isn't sequencing in the classical sense, it's generative composition.

It's not popular because it's much more abstract and harder to work with than "Play this pattern."

There's a lot of unexplored potential in deterministic systems that work with music at these more abstract levels.