As a top-of-the-charts FoL/HL healer, I can tell you that crit is only important in a FoL spec until you get around 45% holy crit. HL healers don't even need crit as their HL's should be non critting for upwards of 16k in ICC gear. Mp5 is nowhere near useless if you know what your stat priorities are.
EDIT: Also, you should be using Glyph of Seal of Wisdom as it is much more efficient than a 16 minute LoH mana gain.