People who use mouse over healing almost always use the party list. In WoW raiding people were generally observed to be faster and more responsive at healing via mouse over macros in comparison to your average person trying to manually target people (via whatever method). Simply put manual targeting means that you are making an additional key press (while mouse over macro means you are shifting your mouse slightly). None the less mouse over healing is personal preference. Most people folks in this game are less inclined to use mouse over healing, partially because of how damage is dealt in this game and due the fact that there are only 8 party members (even for raiding).

Probably the biggest selling point on macro healing is many players are bad at F-keying targets (remembers someones position in the list finding that key and then healing them is not exactly a fast process). Likewise mousing over the party list and selecting a character requires one more click.

Also mouse over healing means you can keep a target in selection at all times (like the boss) making DPS more responsive.