Because PS accounts for a tiny portion of your damage while FF contributes about 6-7% (not counting WP)?

I'm just saying that you don't really much gain anything from drawing one point out of UB and putting it in 0/3 Morbidity to make it 1/3. Same goes for 2/3 and Glyph of disease, how would that 5 sec CD reduce from 3/3 morbidity benefit you in a 20 second rotation? The only real benefit you would draw from it are the 5% increased DC damage. The only scenario where I'd see a use for the CD reduction would be a fight where adds spawn in such a rapid succession (10-15 sec) that you really can't stick to any real rotation at all.

There's really nothing to test about it, as nothing about it is ambiguous. It does exactly what it says, and is a slight upgrade (some of the time) because of that.

There's no "ideal" multi-purpose spec. It's a matter of how much single target dps you're willing to sacrifice for AoE dps, and vice versa. Although, true, there is a point where you can minimize the sacrifice of one while maximizing the sacrifice of the other, it's truly an impossible state to calculate, due to the fact that AoE talents can't be labeled as "X%" dps increases like single target talents.

Ultimately, it's just a matter of opinion. Personally, I would advise simply taking two points from Necrosis and putting them into Morbidity, as that's really all it takes. Yes, that's the same as the AoE spec... but you really can't change anything less. Although, honestly, after running without it for a week, I've found 0/3 Morbidity to work just fine (aside for fights like FC/Gunship, which are laughable anyways), but I'm sure some people wouldn't be able to stand it. Still, on paper, the best balance is simply shifting those two points, to my eyes.

On another note, I corrected the 2P T10 weight in the OP. Although it wasn't wrong before (it was perfectly accurate on how I had done it), I did make an error; I did it using a non-Reaping build. Considering at that point you're changing over to Reaping, that obviously makes no sense, and have corrected it as such. I'll probably change the set table to be identical to the regular stat weight table, in the sense that it's differentiated by tier, as the values change drastically (particularly w/ and w/o Reaping).

Our single target DPS is just fine by what i've experienced so far.

The question here is, do you want to give up comfort for some more dps.
The answer to that is very personal, the dps gain isn't huge; 8% extra white damage minus 10% death coil damage. If your white dps is about 25% of total, and DC+UB about 15% of total, than its 2% dps vs 1.5% dps respectively. (I'm unsure of the actual numbers, but the fact remains the difference isn't large).

This topic is designed to give you the optimal spec for your role. Min/max plays a big role in that and that 0.5% extra dps from my example is 0.5% extra dps. It's for people themselves to make the choice: "Do I want the 10 second reduction on DnD". It's not vital, but it's convenient, because a 20 second cooldown just fits better in your rotation.

It also depends on your current situation; if your guild has everything on farm, than the extra AoE in the greater picture might just speed things up, now that the trash has returned. If your guild however is learning or having trouble on a fight like Saurfang, you might want to opt for that optimal single target spec. It's pure preference and situation specific, its the same question why people still spec DW frost whereas blood and unholy both are better across most fronts.

Consider can't cover all those bases, it's for people's own insight to fill in their spec. And I think it's a bit of a shame most people just take the content of the OP as a golden rule rather than a guideline, if you don't know what your spec is about or fill it in to your own liking you're always missing out an aspect of the game.