Comparing throughput to regen is comparing apples to oranges. A Holy Light Paladin might feel that there isn't such a thing as enough regen, even though most would consider it laughable that a paladin could possibly OOM. Through my gemming and enchanting (and even trinkets), I've chosen more mana. I have when I was gearing up and now that I'm getting more haste, I'm still looking for more.
I picked up the Libram of Veracity once 3.3 hit and I ended up with more triumph badges than you can shake a stick at. I have played with it a little, but for whatever reason I've still only raided with Libram of Renewal. Now I have two Holy Light librams to use situationally, and it means I should probably decide what situational uses this gear has. At first glance, one is for longevity and the other is for throughput.
The very very simplified approach to "which libram is better" is if you run out of mana, you want the mana-savings of the Renewal, and if you don't, why not switch to throughput? I don't think the 'question of OOM' isn't yes or no, each fight will be different. Even if I have a consistent group - the same tanks and healers every week - there are still a lot of variables that can affect my mana usage and my mana regen through a fight, as well as my effective healing and overhealing.
I started going through some math in a recent parse of Lord Marrowgar 25 and Anub 25.
WoL for Anub 25 (farm kill)
WoL for Marrowgar 25 (Our Guild's First ICC Kill!)
For consideration I've posted two logs - one farm status kill and one guild first progression kill. I created a spreadsheet for each with all the mana spent, gained, and saved throughout the fight. For both fights, I was able to regain/save the mana needed to heal throughout the fight. A better way to state that would be "I was able to spend every bit of mana as fast as I was able to gain it back."
Libram of Renewal accounts for 9-12% of my mana efficiency. That's pretty huge for one piece of gear, and I would have to consider other ways to get mana if I wanted to use another Libram. Without a change in playstyle I can not switch to Libram of Veracity without running into mana problems.
However, my playstyle is also built around my having the Renewal (and all other things to make Holy Light fast and plenty). If I were to make a style change and switch to Libram of Veracity, I would have to change a lot of my habits. My Holy Lights overheal for ridiculously embarrassing amounts (58% on Marrowgar, 75.5% on Anub) so the spellpower proc doesn't seem helpful to me. I would have to slow down and shoot for less overheal (probably by casting more Flash of Lights, which could alleviate mana concerns). In other words, the Libram of Veracity could potentially be the strongest in situations where you're casting all three heals on a consistent basis.
When we start discussing playstyle there's a lot of room for opinions and room for growth. This parse shows me just how important the Libram of Renewal is to my healing, and it also points out a couple of really dumb things I didn't do. The biggest hole in these parses is my low uptime of Sacred Shield. On Anub I only casted Sacred Shield twice over a seven minute fight. I am specced for the minute-long duration, but that still leaves at least half the fight where I n00bed it up bigtime. My Marrowgar parse shows me casting four Sacred Shields - again, approximately 3 minutes of the fight where my tank did not have this buff (bad paladin!).
As for more opportunities to regain mana, I wasn't working as hard as I could have. During Marrowgar I could have squeezed in an extra two Divine Pleas that I chose not to (I timed my Pleas with Whirlwind), but on Anub I only hit Divine Plea twice (when Anub burrowed), which left room for at least four if not five more Pleas. I could have also squeezed in a couple more Divine Illuminations, although on a three minute cooldown I might not have been able to do so on Marrowgar (definitely I could have gotten one more use on Anub).
My point is - I can work harder at regaining more mana, but the Libram of Renewal is still always going to be 9-12% of my mana efficiency as long as my primary spell of Holy Light. If you're using this ratio of Holy Lights to Flash of Lights as I am (read: no ratio, just lots of HL), you might find similar results as I have found. In new fights, the tanks (and raid) take more damage now than they ever will (shorter, cleaner fights, more damage avoided, and more mitigation from upgrades). I am more comfortable being armed for the worst-case scenario of damage taken and longevity.
For future runs, I might try some balanced casting (all three heals on a regular basis) to see if I can't get some use out of the Libram of Veracity. It's a major playstyle change for a HL paladin, though, so my progression/pug gear will most definitely still have the Libram of Renewal.