I think you're underestimating Scent of Blood in your guide.
First, we will never have enough RP that you can reserve 20 RP at all time while still bombing DC. That's just not going to happen.
In every pair of runesets (21seconds), we will acquire 110 RP from all our strikes (1PS, 1 IT, 2 DS, 6 HS), with 4 empty GCDs before we have to refresh diseases. If we use just 2 RS every 20s (very likely), we will only have enough RP for 2 DC. Also, reserving 20 RP is a very tricky thing - at least not as simple as you make it out to be. Imagine if you have 45 RP, and the next blood rune will only refresh 2s from now. Do you DC? If you DC, you won't be able to RS for 2s rune refresh + 2 more GCD to strike the runes to get 20 RP enough to RS. But if you don't DC, you're seeing a dip in threat for at least 2s. Without SoB, we will see more GCDs filled up with HoW, which generate very little threat if at all.
Second, don't just look at percentage of recount to determine the value of DC/SoB. That's because: (1) If I remember correctly, the DC proc-ed from Sudden Doom is not able to crit and (2) you're already DCing less in the first place. This goes back to point 1: do you actually DC at every opportunity? If you only do 2 DC at every pair of Runesets, you're potentially only DCing 6-7 times a minute - of course the threat value is very low. If that 6-7x a minute contributes 6-7% of your threat, and if 3/3 SoB can increase the frequency to 9-10x a minute, that is a 3% increase in threat, already equal to the value of Subversion without even counting theoretical higher percentage of RS over melee ratio.
Third, 40 RP per minute per point is an underestimation of its uptime. Its uptime is really high, and it can consume the charge from RS - which can't be blocked/parried/dodged, increasing the RP return from the talent itself. In any multimob tanking (2 and above) you can be guaranteed it has close to 100% uptime. With 3.6 speed weapon and 20% haste, we're expecting to hit a single target boss 20x a minute - with 80% uptime, you're expecting 160 RP per minute.
Now, it's possible that you're not seeing the benefit of extra RP in your raid. If you raid with 1x Disc Priest and 3x Resto Druid bombing you with PW:S and Rejuv, you'll probably have enough RP to the point that you forget having Scent of Blood. Or if you can get 100 RP every 45s from AMS (very possible) you'll probably forget having Scene of Blood. But that's anecdotal - not everyone is going to have that kind of composition, and not every boss has that kind of mechanic.
To prove the point, I created a template in Kahorie's DK Simulator, exactly same spec, gear is what I have in Armory, one with 3/3 SoB and one with 3/3 Subversion. The result is SoB template wins out by 64 TPS (8758 to 8694). No big deal obviously as the difference is <1%, but it shows that SoB is better than "giving more DCs and DCs contributes so little to TPS".