First off, let me say that there are many great ways for us to itemize and choose gear sets. Some people just take what drops, others (like myself) don't roll on it unless its best in slot or a very close 2nd. For those of us who plan our gear sets out, there are some who believe that the 4 piece T7 set bonus is a waste. While in some cases I agree because our T7 is itemized poorly with lots of haste, the set bonuses make up for this. Let me explain how I came to this conclusion.
By using a DPS meter, such as recount, I took the total amount of damage done by judgement and divided it by the amount of casts. While fully raid buffed, for me it averaged out to about 5600 but for arguments sake lets just round it off to 5500. When calculating stats and effects per second, simply divide the cooldown into 60 (seconds). So with Improved Judgements reducing cooldown to 8 seconds, it looks something like this.
60 / 8 = 7.5 x 5500 average damage = 41,250 / 60 = 687.5 DPS
Now to find the damage increase with the 4 set bonus, simply add in a 7 where the 8 is.
60 / 7 = 8.5714 (rounded) x 5500 = 47143 (rounded up) / 60 = 785.7 DPS
So at that point, the average is 98.2 dps increase. Also remember, this scales. The more powerful you become, the more damage this set bonus is worth. Lets not also forget that it will also scale with procs such as the Berserker enchant, Darkmoon Card: Greatness, etc. But since those aren't predictable, lets focus on what we can control, Avenging Wrath. This ability is 20% extra damage with an uptime of 16.65% (20 seconds) if used on cooldown. Lets make this equation simple.
20% damage / 6 (16.65% of 100) = 3.33%.
This is theoretically how much damage that Avenging Wrath is worth to us, providing you use it on cooldown. Now lets see what the numbers look like.
60 / 8 = 7.5 x 5500 = 41,250 / 60 = 687.5 + 3.33% = 710.4 (rounded up)
60 / 7 = 8.5714 x 5500 = 47,142.86 (rounded up) / 60 = 785.7 + 3.33% = 811.88
The DPS isn't the only benefit in the 4 set T7 bonus. I don't know one ret pally who couldn't use some more mana! Judgements of the Wise is worth 659 mana on each judgement. While the extra 1 second seems insignificant on that front, lets check it out.
60 / 8 = 7.5 judgements per minute x 659 mana = 4942.5 mana / 60 = 82.375 mana per second, or 411.875 mana per 5
60 / 7 = 8.5714 judgements per minute x 659 mana = 5648.57 mana / 60 = 94.14 mana per second, or 470.71 mana per 5
So now we've established that the 4 set T7 bonus is also worth 58.835 mana per 5 in addition to the extra DPS. Lets also not forget about replenishment! This may not be a big deal to some guilds depending on your raid composition, but if you have less than 3 players who provide this effect then it does come into play. You'll get 1 extra replenishment effect for the raid every 56 seconds, or 1.07 per minute, with a 15 second uptime.
So this concludes my tutorial. Hope I shed some light on the subject and helped in your choice of whether or not to T7. Also, please note that this is posted PRE 3.0.8, where ret Paladins are getting an extra 6% from Vengeance, and SoM/SoB is getting buffed 5%. While it may go without saying, the 4 piece set bonus will be even more important come the next patch! ENJOY.
*** small addition here. When comparing this set bonus to Elitistjerks.com's stat weights, the 4 set bonus is equivalent to about 55-62 strength. (remember, it scales - but not as well as pure strength. This is merely a weight comparison for those who wish to calculate in stats rather than DPS.) ***
(171 dps per 100 strength is whats posted on EJ's website, 171 / 100 = 1.71 DPS per strength. Rounding down the above mentioned gains to 100 DPS, it would look like this: 100 / 1.71 = 58.48 strength.