First let me say that I am not a huge Addon fan. I have a few but I consider myself a minimalist at heart. Currently I use Auctioneer, Bagnon, Recount, Grid and Clique. These are the few Addons that I have been unable to do without or replace by some function of the default GUI.
I have a Druid, Priest and Pally all in their 80s so I've been around the block when it comes to healing. I don't have a lot of Raid experience (outside of Molten Core) but I've done my share of heroics (haven't we all).
I have heard it all before "If you can't heal without mods then I wouldn't want you healing in my raid..." blah blah blah... Well, I am sorry but using the default interface just limits what you can do. If Blizzard didn't want you to be able to change how targets appear when they were out of range, they shouldn't have built the function into the GUI and made it available to Addons.
The main reason I use Grid is because I can see all the raid targets and know who I can and can't heal/decurse/dispell with 1 single click. I can tell if they are out of range. I can tell if they have a HoT on them. I can even tell if they have aggro. That just can't be done with the default GUI. Cataclysm has brought some improvements on this but it's just not hte same clean interface you get with Grid.
With the default UI raid window sure I can drag groups out. I can even drag classes out to be on my screen. But, I can't tell when they are out of range and I can't tell what buffs/dots they have (without targeting them). Grid allows me to do that very easily. It can even filter DoTs to show only the ones I need to worry about.
Clique allows me to assign up to 8 spells to each mouse button (left or right). At first you want to disable all unit frames aside from the ones associated with Grid (under Clique's options). This way you can still target and right-click targets just like normal and mouse-overs only apply to the Grid.
This gives you an amazing array of options for single click mouse-over casting. You don't have to worry about targeting or even pressing a function key. This can also be done via macros but you have far fewer choices. I can leave my left hand on the keyboard and my right hand on the mouse and heal, heal heal!
I try to make all my actual healing spells different variants of the left-click and utility spells all on the right-click. This way I can have renew, gheal, flash heal, PoM, penance/CoH, and binding heal all on my left-click and buffs, shield, dispell and cure disease on right-clicks.
Couple this ability with Grid's status display and it makes healing very easy and efficient. I am constantly on the top of the healing chart in any BG (unless I defend a tower or something).
Give it a shot. I guarantee you'll be hooked.
Tankspot Grid Tutorial
Basic Grid Options
Shadow Weaving Grid Config
Configuring Grid - Youtube