If I'm casting a heal, I can select my next target and be ready to click on my third by the time the cast is finished. Now, with the haste levels we currently have it doesn't matter all that much anymore because you're spending so long between casts anyway that the time I save by being ready to click on my third target isn't all that much better than moving to my next mouseover target after the 2nd cast has queued, but when casts were all 1-1.1 seconds long it made a difference. In general it lets me map out where I'm going a little better too, but that's just a personal preference.

Tangent'ing off... Mouseover healing also poses a bit of an issue for movement which I don't care for. All the mouseover healers I've known were great throughput healers but were terrible at staying alive. I'm sure there are mouseover healers out there who stay alive just fine, granted, but it is something most healers should consider when going down that road. What do you gain, what do you lose? If mouseover was all pluses, no minuses, every top healer in the world would use it. They don't, and there's a reason for that.