Some of my thoughts on the inlays:
1 - The split inlays are an iconic Chris Reeve design. What Mr. Reeve was thinking with the whole split inlay concept is not known to me but it is a very original look that is unmistakably CRK. I think maybe the original idea of the split inlay design was to mimic the split in the slab for the lock on the back of the knife, thus adding some design symmetry between sides. In my opinion, the split inlay design on the 21 has a more unique appearance than the single inlays on the CRK annuals, regardless of rarity or what you think actually looks better.
2 - The 21 is one of the few knives where you get an inlay on the back (super bonus....thanks Chris!). I.E. there is no "presentation slab" and some boring back side as you find on other decorated frame-locks. With what little real-estate there is on the lock-side, CRK adds the sleek back inlay. This back inlay is a fantastic design element and add to the feeling of "roundness" of the inlayed knives.
3 - The inlays provide yet another perfectly executed manufacturing detail to marvel at. The precision of their fit in their machined pockets is really fantastic. When you are done checking out the other details, you can spend a bit of time fondling the inlays.
In my uses, I don't notice a lot of grip from the inlays. Maybe a bit from the Micarta if kept brand new and grey, but once shiny as smooth, I'm not so sure. In general I find the bead-blasted titanium to be very grippy, so I would only purchase the inlays based on what you think they look like, and maybe to add some bulk to the knife. For the record the wood inlays are very durable. I have two inlayed with wood that I carry regularly and they stand up to abuse, though the Micarta is absolutely bullet-proof.