alanback: "The Crawford game is always cubeless; also, any game that is played when both players are 1 point away from winning the match is cubeless."

That's missing the point, or rather using the wrong perspective. "Cubeless" is a synonym for chequer play. But every backgammon game - in cubeless matches, cubeful games, Crawford and DMP games - has chequer play. However, no cubeless match has the cubing, gammons, game sequence, etc in it. It's the addition of the cube to chequer play that creates the new variation.

There aren't separate ratings among golfers for driving and putting, or among tennis players for serving and volleying;

For sure! There are no putting-only tournaments or driving-only championships. Tennis players don't play three sets of volleying. That's because these things are only parts of their game and are inseparable.

"so why should there be different ratings for cube play and checker play? "

What we are talking about is where an entire match either does or doesn't include a set of features. It's more akin to sprinting and hurdling. Hurdling requires sprinting but not vice versa. They make totally different events, ones which do have separate ratings and rankings. Cubeful Backgammon requires chequer play but cubeless Backgammon is sufficient unto itself. The two are substantially different for all the reasons previously given and are similarly justified in having separate ratings.

"... However, that doesn't mean that separate ratings should be maintained; it just means that a complete backgammon player has to be able to use the cube properly."

This actually highlights another factor in favour of separate ratings: the so called "incomplete" backgammon players! Or are you suggesting that the cube be obligatory and players can like it or lump it?