Just throwing this out there but the 2 reasons Fanaticism are so great are the 2 very reasons on the description.
Ret Paladin wants crits because first off they hit for huge numbers (often more than just 2x damage) and second off the allow other techniques like Vengeance to proc further increasing damage output. The problem is with huge 2 handers hitting so slowly, it's sometimes difficult to work on White damage crits alone. The way to get around this, though, is to use other techniques that have the ability to proc and crit. Using Seals is one great way (SoR, SoC, SoB) all can crit and count towards Vengeance. Judging these seals is another great way to increase the amount of crits your putting out. Certain weapon procs can crit as well and further assist in keeping Crits flowing in regularly. Finally a few direct damage spells (Exorcism, Hammer of Wrath, Crusader Strike, ect) can be thrown in to the mix. With all these techniques able to crit and keep Vengeance fresh, a Paladin can operate with 15% increased damage output from Vengeance indefinately as long as long breaks arn't being taken. Without this Ret paladin would fall far behind Rogues and Mages in the damage department.
Secondly, with huge White and SoC crits going off, agro can be a real problem for Ret Pally who doesn't want it. 5/5 Fanaticism reduces threat generated as if the Paladin was always using Blessing of Salvation, and with Salv on it's much much more. It's a great way to avoid agro when you don't want it.