I think this is a more complicated question than people are considering. I am primarily Holy and play Discipline as my off-spec, which means my set isn't well itemized for Discipline; but still, I seldom have mana issues. Generally, I can hold my rotation up for several minutes before I get low, toss out my Shadowfiend, and then keep going. That is, in general, I don't really think that focusing entirely on the IFSR regen is important. Even though I may be entirely IFSR for long portions of a fight, many fights have lulls or phase transitions where even the most hardcore spammer is afforded some OOFSR regen. Are those transitions and lulls enough to knock Spirit back into a comparable position?

Also, let's not forget that with this buff to MP5 comes a new tier of gear, which means more Intellect which, of course, means that Spirit will be worth more regen in that gear. How much more Intellect can we expect to see on that new gear?

Also, since this a regen question, let's not forget that Intellect is getting it's regen component nerfed with the changes to Replenishment. I haven't run the numbers on it yet, but if MP5 gets buffed enough and Intellect gets nerfed enough, we may see a large change in itemization for regen.