Something that it seemed like worth the math is quantifying the value of extra int given all the 2.4 changes -- I feel like it's now a stat that should be taken rather more seriously.
Int has two benefits: the easy one to model is the increase in one's mana pool, the harder one is the extra spirit regen it provides. (There's also a marginal benefit to one's crit %, but I'll leave that for another day).
For the easy one, each point of int provides 15 mana (16.5 assuming you have kings). Converting that to mp5 is pretty simple: 12 ticks in a minute over x minutes in a given fight gives us (1.375 / x). If you're looking at a shorter fight (say 6 minutes, like brutallus), that's ~.23 mp5 per int. I've often heard it argued that one's mana pool is largely irrelevant based on this since there's only 1 percentage-based mana regen effect (tide), but it's worth noting that if you get an innervate during a fight, you're almost certain to end up topped off (especially with an earring), and thus you
are getting that mana back. (Same thing applies to fights like Illidan with long breaks in the middle that are likely to leave you topped off). If you expect to get mana tide, you'll get an extra 4.125 mana back every 5 minutes (bumping a point of int up to ~.299 mp5), and if you get an innervate halfway through a longer fight, double the effect.
For the harder one, it really boils down to the marginal effect of another point of int on your spirit regen, which depends heavily on your current int and spirit levels.
5 * 0.0093271 * Spirit * Square_root ( Intellect )
So the marginal value of int (in terms of O5SR, assuming that you have kings) is
5* 0.0093271 * Spirit * [Sqare_root( intellect+1.1 ) - Square_root( intellect ) ]
Taking into account the 5SR, assuming 3/3 meditation:
(1-.7*I5SR) * 5 * 0.0093271 * Spirit * [Sqare_root( intellect+1.1 ) - Square_root( intellect ) ]
The algebra, however, isn't all that enlightening. To make it more concrete, take a typical T5/T6 priest with 800 spirit and 650 int who spends 70% of their time I5SR, this gives us approximately .410 effective mp5. Together with the benefits of a larger mana pool on a 6 minute fight, we get something like .64 effective mp5. For comparison, under the same assumptions, a non-human holy priest would get about .7 effective mp5 from 1 point of spirit (taking the difference wrt spirit rather than int). Obviously spirit has other benefits (extra +healing in particular), but from a pure regen perspective, they're reasonably close. If we assume a somewhat higher I5SR time, the values become much closer (.604 for spirit vs. .583 for int @ 80% I5SR for instance).
One somewhat surprising thing to come out of this is that once into T5 level gear, 4 int > 2mp5 in terms of regen under basically any circumstances.
The aforementioned spreadsheet that does most of these calculations is
here. I didn't explicitly include the effect of innervate on the fight's length since you can do it yourself, and I neglected to include mana tide because I'm a slacker. (It's a constant bonus though, so I'm sure you can do it yourself)
One side note to the spreadsheet: I'm aware that blizzard (supposedly) does some really strange rounding in game that might significantly affect the marginal value of stats, but I've never actually seen a listing of how/where they round. If anyone would care to enlighten me (or I missed a posting of it), I'd appreciate it.