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In 6-of-a-suit the king of trumps is arguably as important for maintaining control of the hand as any ace, and it will likely have to be knocked out early if you do not have it. Furthermore, unless you have a favourable finesse, it is an unavoidable loser. That is the main argument for using RKC. I think ace-asking or keycard-asking bids in general are overrated, and you may almost as well do away with most of them. Not much is gained or lost by swapping out one version for a different one.

So 2♦ shows a hand with 2(+) clubs and one of: 4(+)♣, 16(+) hcp and 2(+)♦, or 3(-)♣ (so 2-3), 1(-)♦ and 15(-) hcp, or 3(-)♣ (so 2-3), 2-3♦ and 20(+) hcp.

It might help if you realize that it's not to "find" 12 trick slams. It's to stay out of them! It's not a slam try, as many club players misuse it (and reg blackwood and reg Gerber); it's "I already have high confidence from prior bidding that we have sufficient power/fitting distribution to take 12 tricks + not 2 fast losers in any side suits, but I am just double-checking we aren't off two key cards". These are situations where if you did not have the tool, and your only choices were to bid game or bid six, you'd guess to bid six. But you have the tool, and are trying to cut out bidding six on the ones that are low percentage because of poor trumps, which can be hard to diagnose because most systems don't focus on that in the earlier auction since the only criteria is usually just combined length on most sequences. You really do not want to be off one ace + the K of trumps. In the best case scenario, you have QJT of trumps and it's approx on a hook in trumps. But even those, those are below par, because the opps might have say ace and a ruff if a side suit breaks badly, bringing it slightly below the 50% you need to break even (at either MPS or IMPS). And if you are missing any of Q/J/T, it's much worse, now you need trump K onside, plus maybe a 3-2 trump break, or need both KT onside, or need Kx onside exactly, etc. Or you are missing say K and J and an ace and have zero play. Similar argument, to a lesser extent, applies to missing a key + Q of trumps where in the long run overall you do not want to be in those slams either (although a few you do in retrospect, like slams where the only issue is being off trump KQ with AJT9x vs xxxx, where keycard bidders would avoid but non-kc bidders might bid it) Gerber - other than a few fairly defined sequences, that seem to crop up only once every few years depending on frequency of play, close to 100% of experts do not use it as the common club players who trot it out routinely. 4c is far too valuable to use to show control in clubs or shortness in clubs to give it up for ace asking, you are bidding it when you are still unsure of the power and mesh of the hands for 12 tricks to be there, before you worry about being off two aces, or ace + K trumps, or keycard + Q of trumps.