Spirit was always a good choice for priests, and many priests went for a mix of spirit and mana/5. With the patch, the value of spirit has increased dramatically.
Let's take a step back and look at how mana regeneration works. You have two rates of mana regeneration: one that is "while casting" (referred to as "inside the five-second rule" or is5sr); the other is "out of casting" (a.k.a. "outside the five-second rule"). The "five-second rule" is thus: Your normal spirit-based regeneration stops for five seconds after you COMPLETE casting a spell. (The word "complete" is key here.)
With a 0/0/0 talent spec and no gear, your "out of casting" regen rate is based entirely on your spirit (modified by your intellect; more int = more regen from spirit). Your "while casting" regen rate is zero.
Mana/5 gems and gear are favored by some because mana/5 makes no distinction between "while casting" and "out of casting"; it's the same whatever your situation. For healers in intense end-game situations, trying to find a way outside the five-second rule could be difficult, so some priests fell back to regen they could count on and didn't have to think about.
The trade-off, however, is that Spirit gives much more regen "out of casting" than mana/5 does at the same budget. The distinction, particularly for priests, is large. Spirit gives more oomph, while mana/5 is always on.
It's not that simple, though. Priests have the talent Meditation, which allows 30% of regen from spirit to continue "while casting". This is a considerable chunk of change. It allows spirit to function as mana/5 while still retaining its "out of combat" value. Furthermore, Spiritual Guidance and Improved Divine Spirit give priests spellpower proportional to their spirit, further increasing that stat's value. Certain items (namely the Bangle of Endless Blessings and the Primal Mooncloth tailoring set) allow additional spirit-based regen to continue, skewing things even more towards spirit.
Spirit also stacks particularly well with the talents Spirit of Redemption and Enlightenment, the human racial "The Human Spirit", and the paladin buff Blessing of Kings. All of these things give percentage bonuses to your spirit scores; mana/5 direct from gear is unaffected.
Now, shadow priests are different. Shadow priests are unique in that spirit is invaluable while leveling and almost useless in the end-game. The reasoning is that while leveling, Spirit Tap allows a high-spirit shadowpriest to be a grinding machine. You're guaranteed the killing blow while you solo, so you guarantee yourself a high return from your spirit. At the end-game, in both raids and PvP, being able to get the killing blow is dubious at best. After all, when you're fighting a boss, you don't need to get mana back after the blasted thing is dead; you need it sooner so you can kill him. So a leveling shadow priest relies heavily on spirit; many end-game shadowpriests do everything in their power to divest themselves of the stat.
Is it prudent to have some mana/5? Sure it is; I do, and with the current itemization it's almost unavoidable. Is it wise to favor mana/5 over spirit? I believe the reverse is true. Spirit has made leaps and bounds in recent patches, and should be the healing priest's preferred statistic. All hail the Purified Shadow Pearl!