There was a great failtank story over at Mysterious Buttons where a tank complained about DPS not attacking the right target, yet refused to mark targets. That tank is an idiot, but marking DOES get a little annoying with WoW's default marking system. The "right click someone's profile and select an option" is clunky and awkward. Hell, how many people have invited someone to a group instead of inspecting them? /raises embarassed hand
Not to mention I *may* have once been trying to mark someone in Oculus...and missed my right-click, sending me walking right off an edge... -_-
There's a simple, very basic addon that I use to make marking easy and effortless - QuickMark. [Curse link]
What this addon does is give you a little bar with all the raid marks on them, and you just click an icon to mark your current target with that raid mark. (I just tuck it up at the top of the screen.) You can also change it to a vertical layout to fit your screen layout.
(click to see its size in my mostly-default UI)
Not only does QuickMark eliminate potential profile-click errors, it's also easy and fast, making it possible to raid mark while in combat or while running around. For example, if your group accidentally pulls two groups of enemies, someone using QuickMark can quickly throw up some marks for an impromptu kill order - or heaven forbid, even a CC request.
In short, QuickMark is tiny, useful, and convenient. A great addon.