If there's one thing I don't understand it's the recent 21 utility buzz for mages. The tree is the same as it was in S2, there's nice stuff at the very bottom and at the second level, but the middle is almost completely worthless. It really does not take that much out of your build to pick up something other than Torch for CDR if you want it maxed. @OP: I'd nick a point out of Defense for the Revive mastery until Riot remembers that the tree is worthless outside of SR and moves it back to Defense.

It's better than that. It's more accurate to say it's worth 4-8 AR/MR throughout a game, probably settling on on average around 6. This makes it worth 80 effective HP (4 AR/MR and 1k health, early game) to 400 HP (8 AR/MR and 2.5k health) to more if you're a stacking tank.

The revive boost being 220-560 HP. So it does add more. However, the -damage% mastery is always around, where as the revive boost is going to be in the game for 4 minutes, maybe 6.

Quick copy and paste from spreadsheet, without penetration, rounded to nearest whole number

(Left column = amount of ar/mr, right column = effective ar/mr added by reduction mastery)
15 4
30 4
45 4
60 5
75 5
90 6
105 6
120 7
135 7
150 8
165 8
180 9
195 9
210 10
225 10
240 11
255 11
270 11
285 12
300 12

With mastery and void staff/last whipser

15 3
30 4
45 4
60 4
75 5
90 5
105 5
120 5
135 6
150 6
165 6
180 7
195 7
210 7
225 7
240 8
255 8
270 8
285 9
300 9

You also can't penetrate reduce this effective HP, it is calculated post resistances, and unlike adding flat HP, you can't life-steal it and you're not affected by %HP effects as a result.

I believe you've calculated this incorrectly. The % damage reduction from 21 Defense is multiplicative, not additive. It provides 3% reduction (or about +3.1% effective health) regardless of whether you have 50% or 95% base reduction from armour / MR.

With that said, I did understate the relative importance of penetration. With 40% penetration (VS / LW + pen mastery), the effective value is ~5 armour + MR; add a Black Cleaver and it's about 6. I'd be willing to call it ~5 armour + MR rather than ~3.

In other words, you could roughly calculate it as ~5.3% increase in EH, or around 130 EH @ 2500 champion health. Revive boost will always be considerably heftier, though 100% uptime is a pretty damn good argument for 21 defense. Mmmm. I could go either way.

I know it's multiplicative. You can see how much ar/mr by back calculating.

Here's an example of having 100 ar/mr

100/200=0.5
0.5*.97=.485, so 51.5% is the new amount reduced instead of 50%

100/(100+x)=.485
100=.485(100+x)
51.5=.485x
x=106, so adds 6 AR/MR as our base was 100 armor

If it was additive, it'd really be broken.

Also, penetration makes it worse. The more armor/mr you have, the better it is.