just curious, but since i use grid i haven't ever found a purpose to mouseover casting. i assume about 99% of healers use grid or something like it that makes selecting the target of your heal pretty fast so why mouseover? does it over some other advantage that i'm not aware of?

A few reasons. I don't use anything like Clique and I find mousing over and then pressing the hotkey quicker than clicking and then pressing the hotkey. It means I can target whatever I want and still keep healing. It lets you mouseover a player's avatar rather than unitframe and heal them (although I'll admit this is doubtful at best, as a benefit.) Also it lets you set up strings of modifiers -

#showtooltip Greater Heal
/cast [target=mouseover,help,exists] [target=target,help,exists][target=targettarget,harm,exists] [target=player] Greater Heal (Rank 1)

meaning - please heal my mouseover target if they are friendly, or my friendly target, or my unfriendly target's target, and if none of the above are true, myself.

You can tell how long I've had these macros by the fact that they still have ranks and don't use @target, heh.

Art, even with Clique+Grid, that macro is useful. Clique handles macros as well, and it uses mouseover macros. So you can either key-bind the macro, or plug it into clique. I think you can adapt it to healbot/vuhdo/others as well but it's been so long since I used any of them that I'm not sure.

When I re-read your comment, I realized I didn't answer your question :p

Mouseover casting is one less mouse-click, it's that simple. For those of us who use Clique(me) / Healbot / Vuhdo, we select our target and choose our heal with the same mouse-click. For example, my right mousebutton is mapped to Smite with a version of the macro I posted. My left is mapped to PoM. My sidebuttons are mapped to PW:S and Penance. Other heals are mapped to a modifier+a mouse-button.

For me, if I have to click a target and then click a heal, it would be two separate decisions in my head and thus a longer reaction-time. With the mouse-over method, it becomes one decision for me. It's very much a personal decision where both approaches bring different benefits for different players.