The ordinal designations for CBs (#1, #2, #3), as in some kind of hierarchy of talent/skill level, seem pretty antiquated to me.
These days, most WRs line up all over the place dictating all sorts of adjustable coverages.
Some CBs have elite ability to play LCB (the old-time "#1 CB" role) but can't play RCB and or cover the slot worth a damn. And yet, all three roles are equally crucial to a consistently successful pass defense, and each role requires a somewhat different skill sets and type of athleticism.
So, a guy who primarily plays RCB isn't there instead of at LCB because he's only the "second best" CB on the field. It's because he's the best CB on the field for that spot. And a guy who primarily plays the slot isn't necessarily relegated to that role because he's "not as good as" the guys who primarily cover the boundaries. Many CBs who excel at boundary coverage simply can't cover the slot very well.
The more role versatility a CB has (or any DB, for that matter), the more useful/valuable they are since they're more able to help expand coverage options. It's entirely possible that, although Butler excels at LCB, Gilmore and Rowe may have more role versatility.