Item weights and regen numbers aren't directly compatible. My numbers, barring errors, should tell you which stats yield which results. In this case, "1 MP5" is worth exactly 0.4 MP5, and 1 spirit is worth about 0.35 MP5 at 1293 int. That just tells you which stats are the better at that int level. Assuming I didn't mess up somewhere, 1MP5 is worth 0.40/0.35=14% more than 1 spirit. But 1 Mp5 is also costing 2.5x as much in itemization.
Since manaregen is all that these two stats give a discpriest, they are directly comparable. If 1 spirit is worth 0.22 weight points, then 1 mp5 should be worth 0.22*(2.5/1.14)=0.48 weight points. It's not, so I would assume that EJ calculated these numbers at a much lower intellect level. Doesn't mean they are wrong, just means they might be based on a different premise, or just outdated. I don't know.
Weights can be a good way to determine whether an item is an upgrade or not. If you are unsure of that, but they are pretty dumb when it comes to constraints. If you have an item giving 100 spirit and 20 crit and another item giving 100 int and 20 haste - which is the best?
Item weights might give you that answer. Simply multiply all the numbers with the corresponding weights and you will get two values.
Pick the highest value and you know which item is the better.
But it's not a good benchmark since not all requirements are linear. There are caps, both soft and hard in this game, and item weights don't take those into account. If you have "enough" mana to spam anything but prayer of healing, adding more regen won't do you any good, even if it's the highest rated stat. If you already have 50% crit, should you really put more itemization into crit? Why not increase your spellpower a bit, to make that crit more worthwhile? And if you already are 'hastecapped' for some reason, more haste will only be a waste of itemization. These are things that itemweights cannot help you with.
It's all really depending on which stat you need and how you need them. For holypriests, the need is for some reason almost always manaregen > anything else, but that's completely dependent on playstyle. When i consider an upgrade, I look at the differences in manaregen, spellpower, hp, crit and haste. Sometimes it's worth taking a dip in regen in order to get more crit. Sometimes, it's worth sacrificing an excessive amount of spellpower in order to gain a huge amount of regen. These are choices you usually have to make depending on your own playstyle, item weights aren't really gonna give you a definite answer.
If you just want to see which item is the better in general, itemlevels usually give that answer. For everything else, I much prefer figuring out just what the items are giving me. The addon ratingbuster is excellent at this.