Lesson Learned: Never Macro Your GCD Combos
Sacrificing DPS when you have relatively few keybinds is a big no-no. Never again.
Marking and buffs should be okay, though.
Indeed, not only is dps sacrificed, but macros also have numerous limitations and problems when used to automate GCD combos.
For example, if you are forced to disengage from the boss in the middle of a combo, the macro does not pause the combo until you are in range again. It continues executing the skills in sequence, and when you re-engage, you likely have to start the combo from the beginning again.
As you said, macro for buff and marking is much more useful, depending on the macro of course!
I use Shirk macro that always targets my co-tank (because there is no reason to shirk anyone else), for example, so it saves me from having to select my co-tank manually.
If you do macro your buffs don't be the guy who uses a cheesy /p chat line before the buff. No one needs to know you used Rampart. Same for shirk when your spamming it cause it doesn't always go off immediately.
"(because there is no reason to shirk anyone else)" I beg to differ >B )
Panda to add to that, always use /micon or any /p message at the end of the macro should a message/noise be necessary.
But yea, aside for a aoe/target macro not too much in battle to use a macro for.