^ I agree with you Dillo, in that the added complexity of a retractable tip can present some challenges and compromises. Inevitably there's the firmness issue. I know with my nearly new rOtring 800, it feels reasonably firm, but it's not completely rock-solid. There's a minuscule slight bit of play, not something I'd complain about. I don't know if that'll worsen over time, though. I have a newer 800, which has the plastic sleeve inside the mechanism. It's actually threaded, so if you take it apart you have to be very careful not to strip it. The earlier incarnation was the 600G... retractable like the 800 that followed... and has all metal bits. But try to find an example of one and you'll pay well over $200. That's just nuts. My hope would be to find one that is very badly beaten up with the internals perfectly intact -- just swap inside an 800.
So yes, the sensible thing to do is just supply a cap like the Pentel Sharp Kerry. It looks nicely done with that pen, where the cap posts and a little top comes off to expose an eraser.
I haven't seen the Pilot S20 and S30 models but will have to check them out.