Recently I've seen a few Paladins swapping out their helm and shoulder enchants, switching from Crit to mp5. This has grown legs to the point where some applications specifically request that you make the switch as part of the application process, while others just question your choice - maybe to see if you can defend it.
This of course is the stance I took when I was recently questioned about my choice between the following item pairings.
Arcanum of Burning Mysteries - +30 SP, +20 Crit
Greater Inscription of the Storm - +24SP, +15 Crit
Arcanum of Blissful Mending - +30 SP, +10 mp5
Greater Inscription of the Crag - +24 SP, +8 mp5
Now the SP bonus in each pair is the same, so we'll discard that for now (plus this is a mp5 vs. Crit debate). First, some history.
The great Illumination nerf
Back when Paladins were men, and Arthas was pure, Illumination was an amazing stat. Providing a full 100% mana return on crit, this ability was soon nerfed down to 60% in early Wrath. The powers that be noticed that not only were Paladins not running out of mana, we weren't even having to use Divine Plea. Heck, back in BC I remember watching Reaganomics GAIN mana - while going full bore.
When Illumination was nerfed to 60%, we still didn't notice a discernible impact, so it was further nerfed to 30%. Divine Plea was nerfed to include a 50% healing debuff, and Spiritual Attunement was removed from our kits entirely. Hey, don't blame me that standing in the fire was a valid mana regeneration tool. Even with this, Paladins still have incredible mana regeneration - and a big part of our staying power (even with Blizzard pushing mp5 on us) is from our Illumination returns.
Now for the math
Now, I've covered this before, but here's the basic breakdown for Holy Light - the only spell I'm going to focus on here, as that's our biggest mana hog. Holy Light has a casting cost of 29% of our base mana, which works out to be 1274 mana. Now if you're running the Libram of Renewal, you can take 113 mana right off the top, almost a full 10%. Next, if you're running the Glyph of Seal of Wisdom, you can take another 5% off (maybe it's 5% and then minus 113 - but the numbers are close enough) bringing you down to about 1097 mana per cast. Even with my raid buffed mana pool of 44k+, that can chew a hole in your blue bar in a hurry.
At that cost, our 18 mp5 from above would have to tick for almost five minutes, just to return enough mana for a single cast. Now we're obviously getting mana back from other sources in a raid setting, but we can't always count on those. We also have more mp5 on our gear, and that pushes our total mp5 up some - but the net result from that mp5 remains the same. Five minutes - you get one additional cast. Not many fights last that long.
Our 35 Crit rating equates to approx. .75% crit. Over the course of that same five minute fight, it takes only three of our Holy Lights to crit (that wouldn't have without that extra rating) to generate the mana return of 18 mp5. With a 1.2 second cast time, heck - even if you're only averaging 20 casts per minute (yeah - that's low) you're still bound to pick up three 'extra' crits in that 100 cast time frame.
But the math is wrong!
And you're absoloutely right. If I had no additional crit, and had to choose between running at just .75 crit OR 18 mp5, that mp5 might look a whole lot more attractive. I mean, if every time I push the heal button I have less than a 1% chance to crit, I'm going to be a lot more selective with my casting - and since RNG is Random it's entirely possible to go five minutes without a single crit - even with 50% crit rating.
I have yet to see this happen though (although I have seen some serious dry spells). Looking over a recent WoL though, I had about 2500 healing spells cast (I run about 1:1 HL for FoL over the whole night) and my crit rating was right around 56% - probably a little higher than my actual raid buffed rating - but not by much. This tells me that over time - that rating actually works out. Go figure - Random is Random - but over a long enough period of time, it all comes together.
And your point is?
That's it in a nutshell. Unfortunately your crit rate is subject to the RNG, and mp5 is a static number. You can always count on that 18 mp5 to be there, no matter what you're doing. I personally choose to believe that I'm going to pick up an extra crit every 1.76 min (or 3 every 5 minutes) to compensate for the loss of static mp5. Especially when the time that I need that extra mans is RIGHT NOW when I'm doing my darndest to break my #2 key. I don't need that extra Holy Light 5 minutes from now, I need a whole mess of them between now and then.
Obviously having a good mix of stats is important, but if it's Crit vs. mp5 - I'll take my crit.
Besides, who doesn't love seeing giant green numbers on their screen?