As for most things here, this may have been written out before but haven't seen this around and thought I'd share.

I'm big on a focusless setup. Keeps your focus free. Lots of people who do a focusless setup opt to keep their toons targeting their leader, and then in their macro's use a [target=targettarget] for all spell casts.

Problem I found with this is 3 fold. First, cast sequences do not reset upon your leader picking a new target, meaning that a shadow priest would not re-dot up a new target until it reached the end of the cast sequence and started over. Another problem with this came along with tanking mobs. If I'm tanking, I generally switch around who i'm whacking on. Especially if it's mid pull and I grab an extra few mobs, I'm going to focus on keeping them tanked. With the setup above using targettarget for casts, that means each time my main targets someone new, my DPS switches who their firing at when they really should finish off the mob they are killing. 3rd issue was that for friend spells, I use my tank target for spot healing. If my tank is targeting a friendly unit, that's who the heal will land on. This meant that anytime I needed to spot heal, my DPS lost their target and paused their DPS until I retargeted an enemy mob. All 3 of these frustrated me.

Now, all my DPS spells begin with

Code:
/target [target=target, harm, nodead, exists][target=tank-target, harm, nodead, exists]
This means that as long as the mob they are targeting is an enemy, and alive, they will pew pew it till it's dead. Then, when they are sitting their targeting a dead mob, they will default back onto my tank target, making it their new target to pew pew down. This mean my cast sequences reset and run as intended on mobs, I can spot heal while continuing DPS, and my DPS all focus's on one mob till it's dead before moving onto new mobs. I've yet to run into any issue where this caused problems.

Problems that MAY arrise could be that your guys are stuck targeting something you don't want them to. A fix for this is a simple cleartarget macro letting you in a sense reset their targets thus solving the issue. Hope this helps someone.