Now, hear the opinions of the best healer on the server!!!!
Not wearing plate as a holy paladin is fine but there are several disadvantages.
1) All Cloth and Leather gear have Spirit on it. As a paladin Spirit shouldn't even be something you think about even if it is for the sole purpose of making sure you don't have any. Spirit doesnt give you any buffs and as a Paladin you have such a high mana pool and natural mana regen that you wont need it at all. Mail is normally fine except that mail healing gear tends to lack the fabled Spell Crit you will need as a holy paladin (FoL Bot).
2) Durability. As a paladin healer people expect you to be able to survive anything..but you only have the one bubble every 4 minutes. If a tank sees you being atatcked aswell as a DPS he will be mroe inclined to help the DPS out first and ofc if you cant take the hits, they will blame you
3) Cloth doesnt clank when you walk!
So for the three of these reasons combined always use plate when you can. Only downgrade armour class if there is ARMOUR boost in + healing on the lesser armour class.
I.E. Favor the new PvP healing stuff over cloth/leather PvE blues.
Also, on the subject of healing mods, do not rely on them. They will let you down. I have tried a couple and even in raids like Magtheridon's Lair, the Standard Raid UI is better. The only thing you should have is decurisve, everything else is worthless.
Apart from anything else if you hear/read a healer say "Sorry you died, my mod isn't working"...well, you get my drift.
-- T-Bone